Step 1: Analyze the sentence structure.
The sentence begins with “Patna is not only the capital of Bihar, but it is also one of the oldest cities...”
This construction requires parallelism: “not only X, but also Y.”
Step 2: Check each option.
(A) “capital of Bihar, but it is also one of the oldest” — maintains parallelism, grammatical correctness, and appropriate meaning.
(B) “capital of Bihar, nevertheless also one of the oldest” — incorrect because “nevertheless” breaks the required parallel structure and is less idiomatic here.
(C) “Bihar’s capital city, and it is also one of the older” — uses “older” instead of “oldest,” which changes the intended superlative meaning.
(D) “Bihar’s capital city, but it is also one of the older” — same problem with “older” instead of “oldest.”
(E) “capital of Bihar, but it is also one of the older” — again, “older” is comparative, not superlative.
Step 3: Final Selection.
Only (A) preserves correct grammar, maintains “not only... but also” construction, and uses the correct superlative form “oldest.”
\[
\boxed{\text{capital of Bihar, but it is also one of the oldest}}
\]