Researchers are studying plastics that dissolve at different rates, and they are finding that the so-called "quick disintegration" plastics are taking more time to deteriorate than
they originally seemed.
In the question "than" is a comparative indicator.
LHS to the than: "quick disintegration" plastics are taking more time to deteriorate
RHS to the than: they originally seemed
The comparison is between the actual time taken by the plastics and the time scientists thought that plastics would take.
(A) they originally seemed Incorrect as there is no mention of time
(B) they seemed originally Incorrect as there is no mention of time
(C) it had been thought Incorrect (time had been thought)
(D) it at first appeared Incorrect at first appeared makes no sense.
(E) originally thought Correct (more time to deteriorate than originally thought)