Diwan Sahib was a senior government official in the colonial era of India. He held a high-ranking position in the administrative structure and was responsible for managing various aspects of the government. Here are the details explaining why option 'C' is the correct answer:

1. Definition of Diwan Sahib:
- Diwan Sahib refers to a senior government official who held a prominent position in the administrative setup during the British rule in India.

2. Role and Responsibilities:
- As a senior government official, Diwan Sahib had significant responsibilities in the administration.
- He was involved in decision-making, policy formulation, and implementation of various government initiatives.
- Diwan Sahib oversaw the functioning of different government departments and ensured their smooth operation.
- He played a crucial role in maintaining law and order, collecting revenue, and managing public services.

3. Distinction from Other Options:
- Option 'A': Head of post office - This is not an accurate description of Diwan Sahib's role. His responsibilities were much broader and encompassed the overall governance of a region or province.
- Option 'B': A junior government official - Diwan Sahib held a senior position, so he was not a junior official.
- Option 'D': Head of the bank - While Diwan Sahib dealt with financial matters, his role was not limited to overseeing a bank. He had broader administrative responsibilities.

In conclusion, Diwan Sahib was a senior government official who played a significant role in the administration during the colonial era in India. He was responsible for managing various aspects of the government and ensuring the smooth functioning of different departments.