The correct option is D Subject: Islamabad; Predicate: is the capital of Pakistan
The subject is who or what the sentence is about, and the predicate tells something about the subject. Here the sentence is about Islamabad, and the sentence says that it is the capital of Pakistan. Therefore, Option D is correct. In rest of the options, either the information is taken as the subject(Option A, B) or a part of the information is included along with the object of focus,(Option C).