Assertion (A): Antibiotics do not treat the Corona infection.
Reason (R): Corona is a virus, not a bacteria.

A is true — antibiotics are ineffective against COVID-19.
R is true — SARS-CoV-2 is a virus.
Does R explain A? Yes, and directly: antibiotics work by attacking bacterial structures and processes, which viruses do not have. R gives precisely the reason A holds.
Answer: A is true, R is true, and R is the correct explanation of A.

Assertion (A): Balban could not raise the numerical strength of the army to the level which it reached later at the time of Allauddin Khilji.
Reason (R): Balban insisted upon high birth as an essential qualification for appointment to higher civil and military posts.

A is true — Alauddin Khilji's army was substantially larger than Balban's.
R is true — Balban's insistence on noble birth for high office is well documented.
Does R explain A? Yes. Restricting appointments to men of high birth shrinks the pool available for recruitment, which limits how large the force can grow.
Answer: A is true, R is true, and R is the correct explanation of A.
Note: assertion-reason questions frequently carry subject content — here medieval history rather than logic. The three-step method still applies, but you need the underlying fact to judge steps 1 and 2.

Assertion (A): Nowadays knee pain is a common complaint. Knee replacement is a boon for such sufferers.
Reason (R): Necessity is the mother of invention.

A is true — knee pain is widespread and replacement surgery benefits those affected.
R is true as a general principle.
Does R explain A? This is the interesting part. The proverb might explain why the procedure was developed, but A claims something narrower — that the procedure is a boon to sufferers. A general aphorism about invention does not establish that a specific treatment is effective.
Answer: A is true, R is true, but R is not the correct explanation of A.
The lesson: a proverb or broad generalisation offered as a Reason is rarely the correct explanation of a specific Assertion. When R is much wider in scope than A, suspect option (2).