The correct answer is option 'A' - Alexander's invasion from the northwest.

Explanation:
The decline of the Mauryan Empire was a complex process influenced by various factors. Let's examine each option to understand why option 'A' is not one of the causes.

a) Alexander's invasion from the northwest:
This option is not one of the causes of the decline of the Mauryan Empire. The invasion of Alexander the Great occurred much earlier, around 326 BCE, while the Mauryan Empire was established by Chandragupta Maurya in 322 BCE. The decline of the Mauryan Empire began after the reign of Ashoka, who ruled from 268 to 232 BCE, and Alexander's invasion had no direct impact on the decline.

b) The spread of iron tools and weapons in the outlying areas:
The spread of iron tools and weapons in the outlying areas could have contributed to the decline of the Mauryan Empire. It led to an increase in local power and autonomy, as regions became more self-sufficient in terms of production and defense. This weakened the centralized authority of the Mauryan Empire and made it difficult for the emperor to maintain control over distant territories.

c) The Brahminical reaction against policies of Ashoka:
The Brahminical reaction against the policies of Ashoka is considered one of the causes of the decline of the Mauryan Empire. Ashoka, during his reign, promoted Buddhism and tried to establish dharma (righteousness) as the guiding principle of governance. This led to a decline in the influence and power of the Brahmins, who were traditionally associated with the Vedic religion. The Brahmins, feeling threatened, might have played a role in destabilizing the empire.

d) Financial crisis owing to the maintenance of a huge army:
The financial crisis owing to the maintenance of a huge army was one of the causes of the decline of the Mauryan Empire. The Mauryan Empire maintained a large standing army to protect its vast territory and suppress revolts. The cost of maintaining such a large army, including salaries, provisions, and logistics, put a strain on the empire's finances. This, coupled with declining revenues and increased expenses, led to a financial crisis that weakened the empire's ability to govern effectively.

In conclusion, the correct answer is option 'A' - Alexander's invasion from the northwest, as it did not directly contribute to the decline of the Mauryan Empire.

The decline of the Mauryan Empire was a complex process with multiple factors contributing to its downfall. However, out of the options given, the invasion by Alexander from the northwest is not considered one of the causes of the decline.

Explanation:
1. Financial crisis owing to the maintenance of a huge army: The Mauryan Empire maintained a large army which required significant financial resources for its upkeep. The burden of maintaining and paying for this army led to a strain on the empire's finances, contributing to its decline.

2. The spread of iron tools and weapons in the outlying areas: The spread of iron tools and weapons in the outlying areas actually played a role in the expansion and consolidation of the Mauryan Empire under Emperor Ashoka. Iron tools and weapons helped in agricultural productivity and military strength, which were crucial in the empire's growth. Therefore, this factor cannot be considered a cause of decline.

3. The Brahminical reaction against policies of Ashoka: After Emperor Ashoka embraced Buddhism and promoted its principles, there was a backlash from the Brahminical section of society. The Brahmins, who held significant religious and social influence, opposed Ashoka's policies, leading to a decline in his popularity and weakening of the Mauryan Empire.

4. Alexander's invasion from the northwest: This option is not a cause of the decline of the Mauryan Empire. Alexander the Great's invasion of India occurred in the 4th century BCE, which was much earlier than the Mauryan Empire's existence. The Mauryan Empire was established by Chandragupta Maurya in 322 BCE, after Alexander's invasion. Therefore, Alexander's invasion did not directly contribute to the decline of the Mauryan Empire.

In conclusion, out of the options given, the invasion by Alexander from the northwest is not one of the causes of the decline of the Mauryan Empire. The decline was primarily influenced by financial crises, Brahminical reactions against Ashoka's policies, and internal administrative issues.