A constitution that requires a plebiscite or a referendum to be amended is

A.

rigid

B.

unwritten

C.

flexible

D.

written

Correct answer is A

Plebiscite; the direct vote of all the members of an electorate on an important public question such as a change in the constitution.

A referendum; is a direct and universal vote in which an entire electorate is invited to vote on a particular proposal and can have nationwide or local forms.

A rigid Constitution;  is a constitution that is difficult or slow to change usually because of a prescribed process of amendment that is detailed and lengthy in execution.