A. I have been writing a letter.
B. I has written a letter.
C. I had written a letter.
D. I have written a letter.
Given sentence is in present perfect tense and it is in passive voice.
Rule :
Subject + (have/has) + V3 + Other agents
A. I have been writing a letter.
B. I has written a letter.
C. I had written a letter.
D. I have written a letter.
Given sentence is in present perfect tense and it is in passive voice.
Rule :
Subject + (have/has) + V3 + Other agents