The correct answer is: If.
The sentence is about a hypothetical situation, so we need to use a conditional conjunction. “If” is the most common conditional conjunction, and it is used to express a condition that is necessary for something else to happen. 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 express a condition. “Unless” is used to express a condition that is necessary to avoid something else happening. “When” is used to express a time when something happens. “Hardly” is used to express surprise or disbelief.
Here are some examples of how the other options could be used in a sentence:
- Unless Salman had married in time, his kids would have been born out of wedlock.
- When Salman married, his kids were already teenagers.
- I can hardly believe that Salman is still single.