Hi Abhishek!
I will go with soft delete approach. With this, you can keep a log of questions that have been asked so far -- including the ones that have been deleted. Questions get deleted for varieties of reasons such as spam reports, the OP decides to delete and so on. But you should be able to answer questions such as how many questions came into the system, how many got deleted and for what reason etc. So, it's best to soft delete them even though it results in higher data size.
Joseph S Stevens
10 years of Software Development, Husband, Father and Functional Programming enthusiast.