You must either (1)/ be regular with your studies (2)/ and study for longer period before the ex amination. (3)/ No error (4)

Answer

3

Explanation

or will replace and because -
either - or, neither-nor are correct forms of Correlative Conjunctions, used for showing the relationship between two things or two terms that are different from each other.
and (Conj.) is used for showing the relationship between two things/situations that exist together. Look at the examples given below:
Either he could not come or he did not want to. I.m going to buy either a scooter or a motorcycle. Do it slowly and carefully.
She came in and took her coat off.
Hence, or study for longer period before the examination is the right usage.