Explanation:

Correct answer is (a). The two sentences given above present a contrastive sense and thus usage of ‘Besides’ or the conditional clause introducer ‘if’ would be irrelevant in the context.
Let’s now check the relevance of the option A here.
Possibility:
While many in the strategic community rightly blame politicians and bureaucrats for their reluctance to appoint a Chief of Defence Staff, they often overlook resistance to jointness within the services.
Clearly, the above synthesis is both grammatically and contextually correct and keeps the flow intact.
Option A is hence the correct answer.