Hippopotamuses allow their hides and teeth to be cleaned by carp, members of the genus labeo, which, in turn, benefit by eating the algae and parasites on the giant mammals.
(A)
which, in turn,
benefit by eating the algae and parasites on the giant mammals
which, properly refers to to subjuct + Proper use of "in turn" + Subject-Verb Agreement.
(B) which
benefits by eating the algae and parasites on the giant mammals in turn
Subject-Verb Agreement, carp is plural as specified by the use of "
Members of the genus labeo"
(C)
benefiting, in turn, by
eating the algae and parasites on the giant mammals
Allow + Cleaned =/= benefiting + eating
Not Parallel (D)
to benefit, in turn, by eating the algae and parasites on the giant animals
"to benefit" in ungrammatical and skews the meaning
(E) who
benefits by eating the algae and parasites on the giant mammals in turn
Subject-Verb Agreement, carp is plural as specified by the use of "
Members of the genus labeo"