I have not written any letter to him since my father had died
Ans:1. was dead 2. died 3.has died 4.no improvement

Question

I have not written any letter to him since my father had died 
Ans:1. was dead 2. died 3.has died 4.no improvement

Answer

3. has died. 
An action happened at an unspecified time before now. The exact time is not important.

I have not written any letter to him since my father had died Ans:1. was dead 2. died 3.has died 4?