Question

give him a suitable reply change into passive voice ​

Answer 1

Passive voice: A suitable reply must be given to him.

Rules to change Active voice into Passive voice:

Active Voice                                                Passive Voice

Subject + V1+s/ es + object   ---->             Object+ is/am/are+V3+by +subject

Subject + Do/does+ not + V1 + Object ---> Object + is/am/are+ not + V3+ by Subject

Does+ Subject+ V1+Object + ? ---> Is/am/are + Object+ V3+ by subject + ?

The above sentence is a declarative sentence thus we have used 'must.'