The poet read his long poem himself.

A. The long poem was read by the poet himself.
B. The long poem read by the poet himself.
C. The long poem had been read by the poet himself
D. The long poem is read by the poet himself.

Given sentence is in simple past tense and it is in active voice, we need to change it into passive voice.

Rule :
Subject + (was / were) + V3 + Optional Agents.