Sentence improvement — future time clause after 'until': use present simple for future reference and write 'until they arrive', not perfect/future forms.

Given sentence
His father won't be able to leave for Varanasi until they have arrived.

Concept/Approach
In time clauses introduced by 'until/when/after/before', English uses the present simple to refer to future time. Therefore, prefer 'until they arrive' (not future or perfect forms) when the arrival is simply a future event.

Correction
'have arrived' → arrive

Why others are wrong
'will have arrived'/'will arrive' → future tenses are not used in subordinate time clauses.'No improvement' → keeps a tense mismatch.

Final Answer
until they arrive