Assertion (A) : The Covid-19 virus has a shorter life-span and evolves into new strains at a fast speed.

Reason (R) : RNA being unstable, mutates at a faster rate.

The correct option is: Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

Let us break down the scientific reasoning behind this step-by-step:

1. Analysis of Assertion (A):
The Covid-19 virus (SARS-CoV-2) is a single-stranded RNA virus. Like many other RNA viruses, it exhibits a relatively short life-span outside host cells and replicates rapidly within host cells. During rapid replication, the virus frequently mutates, which leads to the continuous emergence of new variants or strains (such as Delta, Omicron, etc.) at a fast speed. Therefore, Assertion (A) is a true statement.

2. Analysis of Reason (R):
Genetic material can be either DNA or RNA. Chemically, RNA is much more unstable compared to DNA. The presence of a 2'-hydroxyl group (-OH) in the ribose sugar of RNA makes it highly reactive and prone to degradation. Additionally, RNA polymerases (the enzymes that copy RNA) generally lack proofreading mechanisms, which leads to frequent copying errors during replication. As a result, RNA mutates at a significantly faster rate than DNA. Therefore, Reason (R) is also a true statement.

3. Link between Assertion and Reason:
The fundamental reason why the Covid-19 virus evolves into new strains at such a fast speed (as stated in Assertion A) is that its genetic material is RNA, which, being chemically unstable and lacking robust proofreading during replication, mutates at a very rapid pace (as stated in Reason R). Thus, Reason (R) directly explains why Assertion (A) occurs.