Sunita decided to set . . . . . . . . some time everyday for prayers.

The correct answer is D. aside.

To set aside something means to keep it for a particular purpose. In this case, Sunita is setting aside some time every day for prayers. This means that she is making time for prayers and is not allowing other things to interfere with that time.

The other options are not as appropriate in this context. To set up something means to put it in place or to establish it. To set in something means to put it into something else. To set on something means to put it on top of something else.

In this case, Sunita is not putting prayers in place or establishing them. She is not putting them into something else or putting them on top of something else. She is simply setting aside some time for them.