It is better to study on morning or evening?

Whenever you want to. If you want to study in the morning to get it over with then you can but if you are busy during the day then you can study at night. Whenever you want to.

It all depends on when you are most awake, alert, energized and able to successfully concentrate or focus attention. Studying when tired is unproductive. Some people are morning people and others are night people. Observe throughout each day when you feel most able to focus attention and can easily retain information, that will be the most optimal and productive time to devote to studying. If that time is already devoted to something else, then study during the next best time of day when you feel you are able to be productive and make progress with retaining information and can easily sit and think.