Question

you ____ attend the class regularly .(necessity)​

Answer 1

Answer:

you not attend the class regularly

Answer 2

Answer:

You must attend the class regularly.

Explanation:

Since the need for necessity is emphasized we shall use must.

Must is a stronger word than should.

Whereas,

Should is used to tell somebody what they ought to do, to give advice, or to add emphasis to something important.

Given the requirement of the question we will must.