Step 1: Reference from the poem.
The poem clearly mentions that the dust of snow fell on the poet from a hemlock tree, which indicates the poet was standing under that tree.
Step 2: Analyzing the options.
(A) Under a neem tree: Incorrect, not mentioned in the poem.
(B) Under a hemlock tree: Correct — directly mentioned in the poem.
(C) Under a peepal tree: Incorrect, not part of the text.
(D) None of the above: Incorrect, since the right option is already provided.
Step 3: Conclusion.
The poet was standing under a hemlock tree, so the answer is (B).