The correct answer is C. If.
The sentence is in the conditional form, which is used to talk about things that are possible or probable. The word “if” is used to introduce the condition, and the word “would have been” is used to talk about the result of the condition. In this case, the condition is that Salman had married in time, and the result is that his kids would have been adult till now.
The other options are not correct because they do not make sense in the context of the sentence. The word “hardly” means “almost not at all,” and it would not make sense to say that Salman had almost not married in time. The word “when” is used to talk about things that happen in the future, and it would not make sense to say that Salman’s kids would have been adult in the future. The word “unless” is used to talk about things that are possible but not certain, and it would not make sense to say that it is possible but not certain that Salman had married in time.