Bengal nawabs were the rulers of the Bengal Subah, which was a province of the Mughal Empire in the Indian subcontinent. They held significant power and authority during their reign. However, none of the given options accurately describes the true characteristics of the Bengal nawabs. Let's analyze each option in detail:

a) They had strong armies.
This statement is not entirely accurate. While it is true that the Bengal nawabs maintained armies, it would be misleading to say that they had strong armies. The Bengal nawabs were often vulnerable to invasions and faced challenges in maintaining a formidable military force. The Mughal Empire was weakening during this period, and the Bengal nawabs often had to rely on alliances with regional powers to defend their territory.

b) They firmly tried to put down the use of military force by the English East India Company.
This statement is not true. The Bengal nawabs did not try to put down the use of military force by the English East India Company. In fact, the nawabs were initially tolerant towards the English traders and granted them trading privileges. However, as the East India Company's power and influence grew, tensions arose between the nawabs and the Company, leading to conflicts such as the Battle of Plassey in 1757.

c) They checked growing corruption among their officials.
This statement is also not true. The Bengal nawabs did not effectively check the growing corruption among their officials. The administration under the nawabs was marred by widespread corruption and inefficiency. The officials often exploited their positions for personal gain, which had a detrimental impact on governance and the welfare of the people.

d) None of these.
This is the correct answer. None of the given options accurately describes the true characteristics of the Bengal nawabs. The nawabs faced various challenges during their rule, including weakening central authority, regional conflicts, and the encroachment of European powers like the English East India Company.

In conclusion, none of the given options accurately describes the true characteristics of the Bengal nawabs. They did not have strong armies, did not attempt to curb the use of military force by the English East India Company, and were unable to effectively check corruption among their officials.