1. ............ is an officially compulsory, or otherwise considered essential; indispensable.

Requisite

Required

Requisition

Requite

2. ............ is a synonym of required.

Compulsory

Voluntary

Inessential

Optional

3. ............ isn't a synonym of required.

Compulsory

Needful

Deserved

Optional

4. Add seasoning, ............ required.

so

as

of

for

5. Quotation : we will reply you in the ............ time

require

requires

required

requiring