The following element of syntax appear in both poems:
D. Enjambment.
Both poems present enjambment, as one can see on the first quatrain of "Imagery" or the verse that begins with "But to me..." in the case of "Summer Rain".
That said, one can argue that other elements are present on both poems.
In both poems, there are paralell structures. In "Summer Rain", on the lines that include verbs in the past tense ("rattled and clashed"; "danced and glittered") and in "Imagery", MacLeish uses it on "bosomed" and "blossomed".
One could argue as well the presence of anaphora, with the different repetitions on both poems. In "Summer Rain", the three "And"s, while "Imagery" has "All".
In any case, these other elements appear in a limited manner, so the answer is still D as enjambment is the primary element that is shared between poems.