The correct answer is option 'D', Awadh.

Awadh, also known as Oudh or Ayodhya, was referred to as the 'Nursery of Bengal Army' during the colonial era. It played a significant role in the recruitment and training of soldiers for the Bengal Army, which was one of the main military forces of the British East India Company.

Here is a detailed explanation of why Awadh was called the 'Nursery of Bengal Army':

1. Historical Background:
- Awadh was a princely state located in the northern part of India, with its capital in Lucknow.
- It was ruled by the Nawabs of Awadh, who were initially appointed as governors by the Mughal Empire but later gained autonomy.
- The Nawabs of Awadh maintained a well-organized and disciplined army, which attracted the attention of the British East India Company.

2. Recruitment Base:
- Awadh had a large population and a reputation for producing skilled and brave soldiers.
- The fertile land of Awadh provided the economic stability necessary for the maintenance of a standing army.
- The Awadhi people, known for their martial traditions, were recruited into the Bengal Army in large numbers.

3. Training Centers:
- Awadh had several training centers and military schools where recruits were trained in military tactics, weaponry, and discipline.
- The British East India Company established military cantonments and barracks in places like Lucknow, Faizabad, and Ayodhya to train and house the soldiers.
- These training centers were equipped with modern weapons and were supervised by British officers.

4. Contribution to the Bengal Army:
- The Bengal Army was the largest and most important of the three armies maintained by the British East India Company.
- Awadh played a crucial role in supplying a significant portion of the soldiers for the Bengal Army.
- The soldiers from Awadh served in various military campaigns and wars, both within India and outside.
- The Awadhi soldiers had a reputation for their loyalty, discipline, and bravery, which made them highly valued by the British.

In conclusion, Awadh was called the 'Nursery of Bengal Army' because of its significant contribution to the recruitment and training of soldiers for the Bengal Army. The large population, martial traditions, and well-established training centers in Awadh made it an ideal source for the army's manpower needs.