The correct answer is option 'C' - Kayasthas.

Explanation:

Historically, the Brahmins held a dominant position in the Indian social hierarchy as the highest varna (or caste) and enjoyed significant power and privileges. However, during the post-Gupta period, a new social group emerged that started competing with the Brahmins for power and position - the Kayasthas.

1. Introduction to the post-Gupta period:
- The Gupta Empire, which flourished from the 4th to 6th centuries CE, was known as the "Golden Age" of India.
- After the decline of the Gupta Empire, India entered a phase of political fragmentation and regional kingdoms.

2. Rise of the Kayasthas:
- The Kayasthas were a social group that emerged during this period and gained prominence in various parts of India.
- The exact origin of the Kayasthas is debated, but they are believed to have emerged from a mixed Brahmin-Kshatriya lineage.
- Initially, the Kayasthas were primarily engaged in administrative and bureaucratic roles, serving as scribes, accountants, and record-keepers in the royal courts.
- Over time, they gained significant influence and power due to their proximity to the ruling elites.

3. Competing with the Brahmins:
- The Brahmins, as the traditional priestly class, held a monopoly on religious and ritual practices.
- However, the Kayasthas began to challenge the Brahmins' position by acquiring knowledge of religious texts and rituals.
- They started performing priestly functions, such as conducting ceremonies and rituals, which were traditionally the domain of the Brahmins.
- This led to a competition between the Kayasthas and the Brahmins for power and position in society.

4. Reasons for the Kayasthas' rise:
- The decline of the Gupta Empire resulted in a power vacuum, creating opportunities for other social groups to assert their influence.
- The Kayasthas capitalized on their administrative skills and proximity to the ruling elites to establish themselves as a powerful group.
- Their role as record-keepers and administrators gave them access to resources, wealth, and patronage, enabling them to rise in social status.

5. Impact on the social hierarchy:
- The emergence of the Kayasthas as a powerful social group challenged the traditional Brahminical hierarchy.
- It signaled a shift in power dynamics and the diversification of the ruling elite.
- The Kayasthas' rise also reflected the changing social and political landscape of medieval India, where administrative skills and proximity to power became valuable assets.

In conclusion, during the post-Gupta period, the Kayasthas emerged as a social group that competed with the Brahmins for power and position. Their administrative skills and proximity to the ruling elites allowed them to challenge the traditional dominance of the Brahmins in society.