Solution Explanation

This question requires evaluating two statements about microbial metabolism, specifically concerning nitrate utilization under anaerobic conditions.

Statement Analysis

• Assertion (A): Some microorganisms can use nitrate as a terminal electron acceptor under anaerobic conditions.
  • This statement is correct. Many facultative anaerobic bacteria can switch from aerobic respiration (using oxygen) to anaerobic respiration when oxygen is absent. In this process, they use alternative electron acceptors, such as nitrate ($NO_3^-$), to generate energy. This is a key aspect of the nitrogen cycle.
• Reason (R): Nitrate can be fully reduced to ammonia in the process of denitrification.
  • This statement is not correct. The standard definition of denitrification involves the microbial reduction of nitrate ($NO_3^-$) through intermediates like nitrite ($NO_2^-$), nitric oxide ($NO$), and nitrous oxide ($N_2O$), ultimately resulting in the production of nitrogen gas ($N_2$). While some microorganisms can reduce nitrate to ammonia ($NH_3$) via a process called Dissimilatory Nitrate Reduction to Ammonia (DNRA), this is typically considered a separate pathway from denitrification, which produces $N_2$. Therefore, denitrification does not typically result in the full reduction of nitrate to ammonia.

Conclusion

Since Assertion (A) is factually correct and Reason (R) inaccurately describes the primary outcome of denitrification, the most appropriate answer is that (A) is correct and (R) is not correct.