The correct answer is: The poet read his long poem himself.
The other options are incorrect because they do not accurately reflect the tense of the sentence. The sentence is in the simple past tense, which means that the action of reading the poem happened at a specific point in time in the past. Option A, “The long poem was read by the poet himself,” is in the passive voice, which means that the focus of the sentence is on the action of reading, not on the person who did the reading. Option B, “The long poem read by the poet himself,” is in the simple present tense, which does not match the tense of the original sentence. Option C, “The long poem had been read by the poet himself,” is in the past perfect tense, which means that the action of reading the poem happened before another action in the past. This does not match the context of the original sentence, which does not mention any other actions that happened before the poet read his poem.
In conclusion, the correct answer is: The poet read his long poem himself.