A 1972 agreement between Canada and the United States reduced the amount of phosphates that municipalities had been allowed to dump into the Great Lakes.
This is a very tricky sequence of tense question and we need to focus on meaning then only we can conclude correct tense.
Point to be noted:
Discharge or dumping was reduced not totally disavowed. It is still permitted, but in diminished proportions.
had is absolutely wrong, because discharge though diminished still continues.
A and B are out.
Lets move ahead and see how C, D and E comes out to be.
reduced vs reduces split
1972..agreement... we need a past verb form. reduced is correct, reduces is incorrect.
C and E goes away.
D is correct. "reduced the amount of phosphates that municipalities are allowed to dump" are is correct as municipalities are still allowed to dump/discharge, though proportion is diminished.
2 Explanations