Let’s sort this question out by analyzing splits.
1. Split:
but also /
he also /
alsoa. What if we say Admiral not only had a brilliant career... but also had the distinctive stint... the idiom under revision is not only X but also Y. For the sake of parallelism we need X to be parallel to Y. X (had) = Y (had). The verdict is that the idiom is used correctly with parallel X and Y.
b. What if we say
Admiral not only had a brilliant career... he also had the distinctive stint... MGMAT SC guide says that we can say
not only X but also Y /
not just X but also Y /
not only X but X. However, saying
not only X also Y is wrong. Thus D is out.
c. What if we say
Admiral not only had a brilliant career... also had the distinctive stint.... Once again, saying
not only X also Y is wrong. Thus E is out.
2. Split:
serving as the first /
serving as only the first /
serving as the first one onlya. What if we say
Admiral served as the first Governor-General of Independent India. The meaning is clear and
the first is parallel to
the last. Fine.
b. What if we say
Admiral served as only the first Governor-General of Independent India. This sentence implies that admiral could, if he wanted, serve both as the first and the second or whatever number Governor-General of Independent India. We can’t say that we were both the first and the second to cross the finish. We are either the first or the second. Thus saying that we crossed the line
only the first is illogical. B and E are out.
c. What if we say
Admiral served as the first one only Governor-General of Independent India. The meaning seems to be admiral was the first and the only governor, and there was no governor after him. In other words, he was both the first and the last governor. I am not sure whether this info is correct. Anyways, we don’t need
one over there. C is out.
Hence
A