Unless Geeta apologises (1)/ she should not be (2)/ allowed to go to picnic. (3)/ No error (4)

Answer

3

Explanation

It is a Preposition related error. Hence, allowed to go for a picnic (Noun) is the right usage.
Look at the sentences :
Let.s go for a picnic.
They should go on a picnic.
Both on and for can be used. But, most commonly used is for.