When a bill is referred to a joint sitting both the Houses of the Parliament, has to be passed by

The correct answer is a simple majority of member present and voting.

According to Article 108 of the Constitution of India, if a bill is referred to a joint sitting of both Houses of Parliament (the Lok Sabha and the Rajya Sabha), it is resolved by the members present and voting at that joint sitting.
A joint sitting is convened by the President to resolve a deadlock between the two Houses over the passage of a bill.

At such a joint sitting, the bill is passed if it is supported by a majority of the total number of members of both Houses present and voting. This is referred to as a simple majority.
Since the Lok Sabha (Lower House) has a much larger membership compared to the Rajya Sabha (Upper House), the perspective of the Lok Sabha usually prevails in a joint sitting, which requires only a simple majority of those present and voting to pass the bill.