This letter will reach him (1)/ before he left (2)/ for Delhi. (3)/ No error.(4)

Answer

2

Explanation

leaves will replace left because -
When the tense in the Main clause is will/going to (referring to the future), the tense in the Conditional Clause will be in Simple Present.
This letter will reach him `rarr` Main clause
Before he `underset("Simple Present")underset(darr)("leaves")` for Delhi `rarr` Conditional Clause
Hence, before he leaves is the right usage.