The correct answer is option 'A', Shakti.

Explanation:
The four varnas are an ancient system of social classification in Hindu society. They are based on occupation and are believed to have originated from the Hindu scriptures known as the Vedas. The four varnas are:

1. Brahmins:
Brahmins are the highest varna in the social hierarchy. They are traditionally priests, scholars, and teachers. They are responsible for performing religious rituals, preserving knowledge, and guiding the society in matters of spirituality.

2. Kshatriyas:
Kshatriyas are the warrior or ruling class. They are responsible for protecting the society, maintaining law and order, and governing. Traditionally, they held positions of power and authority, including kings, warriors, and administrators.

3. Vaishyas:
Vaishyas are the merchant and agricultural class. They are involved in trade, commerce, farming, and business activities. They play a crucial role in the economy by producing and trading goods.

4. Shudras:
Shudras are the laborer or servant class. They are engaged in manual and menial tasks, providing services to the higher varnas. They do not have the same privileges and opportunities as the other varnas.

It is important to note that Shakti is not one of the varnas. Shakti refers to the divine feminine energy or power in Hinduism. It is associated with the goddesses and represents the creative and nurturing force in the universe. While Shakti is a significant concept in Hindu mythology and worship, it is not considered a social classification like the varnas.

In conclusion, the correct answer is option 'A', Shakti, as it is not one of the four varnas in the traditional Hindu social system.