Heres my reasoning. Open for correction and deliberation.
A.
and a cereal that is used to make alcoholic drinks,
have decreased over time because of agricultural practice, and to maintain the diversity of the crop- diversity requires singular verb. "and" before modifier is incorrect.
B.
a cereal is used to make alcoholic drinks,
have decreased over time because of agricultural practice, and maintaining the diversity of the crop - modifying phrase contains a main verb "is", SVA issue.
C.
a cereal that is used to make alcoholic drinks,
has decreased over time because of agricultural practice, and to maintain the diversity of the crop - Correct
D. a cereal that is used to make alcoholic drinks,
was decreasing over time because of agricultural practice, and to maintain the diversity of the crop- use of "was" is incorrect. The action continues to the present and future action is required.
E.
and a cereal that is used to make alcoholic drinks, has decreased over time because of agricultural practice, and to maintain the diversity of the crop - modifying phrase is incorrect which doesnt make sense.