Step 1: Background of Bengal Army.
The Bengal Army was one of the three Presidency Armies of the East India Company (along with Madras and Bombay Armies). Soldiers were mostly recruited from North India.
Step 2: Why Awadh was important.
Awadh (Oudh) was rich in manpower. The British recruited large numbers of peasants and soldiers from Awadh into their army. Because of this, Awadh was described as the “Nursery of the Bengal Army.”
Step 3: Conclusion.
Thus, the region of Awadh became the primary recruiting ground for the Bengal Army.
\[
\boxed{\text{Correct Answer: Awadh}}
\]