The speaker of the poem is sometimes the poet. Thus, the correct answer is option D.
What is a poem?
A poem is a piece of writing in which the words are carefully chosen for their beauty and sound and are carefully arranged, often in short rhymed lines.
A poet is someone who researches and writes poetry. Poets may self-describe as such or be described as such by others.
Assuming that the poet or writer is always the speaker of the poem may be incorrect because the poet may detach himself/herself from the speaker's voice and convey the message through a different person in a variety of situations. However, when it comes to representing specific emotions and personal views, poets can also be the speakers of the poem.