Question

باید توجیه بی عیب و نقصی داشته باشی

Answer 1

You must have a/an faaultless/correct/adequate/right explanation/justification.

Answer 2

You must have / provide us with a sound / perfect / cogent justification. / argument.

cogent

if a statement is cogent, it seems reasonable and correct.

a cogent argument for banning the drug (Webster)
 

 

 

Answer 3

Flawless justification

Answer 4

an airtight alibi

Lucky for him, his alibi is watertight.