Assertion (A) : Biologists are sure about how many prokaryotic species are living now.

Reason (R) : The conventional taxonomic methods are not suitable for identifying microbial species.

The correct option is: Assertion (A) is false, but Reason (R) is true.

Let us break down the logical reasoning step-by-step to understand why this option is correct:

1. Analyzing Assertion (A):
Assertion (A) states: "Biologists are sure about how many prokaryotic species are living now."
This statement is false. In reality, biologists are highly uncertain about the actual number of prokaryotic species currently living on Earth. The vast majority of prokaryotes (bacteria and archaea) have not yet been discovered, described, or cataloged. Estimates of their total diversity range from millions to potentially billions of species, meaning we are far from having a definitive count.

2. Analyzing Reason (R):
Reason (R) states: "The conventional taxonomic methods are not suitable for identifying microbial species."
This statement is true. Conventional taxonomic methods typically rely on culturing microorganisms in a laboratory and studying their morphological, physiological, and biochemical characteristics. However, the vast majority of microbes (estimated at over 99%) are "unculturable" under standard laboratory conditions. Because they cannot be grown and isolated, conventional methods fail to identify or classify them. Modern biology relies on culture-independent molecular techniques, such as metagenomics and sequencing of 16S rRNA genes, to identify these microbial species.

Conclusion:
Since biologists cannot easily culture and identify most prokaryotes using conventional methods, they remain unsure about the total number of prokaryotic species. Thus, Assertion (A) is false, and Reason (R) is true.