Explanation:

None of the above

Central government was able to generate an adequate amount of revenue but was not able to spend it properly. (Incorrect - The issue during the economic crisis of 1991 was more about the fiscal deficit and the inefficient allocation of resources rather than the government's ability to generate revenue.)

Widespread public sector economy was profitable but heavy corruption was there. (Incorrect - The public sector was facing inefficiencies, and corruption was a concern, but it was not the sole reason for the economic crisis.)

A large share of foreign exchange was spent on the defense sector. (Incorrect - While defense expenditure was a factor, it was not the primary reason for the economic crisis.)

Export of Indian products exceeded its import to the international market, but it sold at a lower price due to its poor quality. (Incorrect - The balance of trade was a concern, but poor quality alone was not the primary cause of the economic crisis.)

So, the correct answer is "None of the above," which corresponds to option 4.