In the face of widespread concern about environmental waste, compact disc manufacturers are attempting to find a replacement for
the disposable plastic box in which they package their product.
(A) the disposable plastic box in which they
Correct.(B) the disposable plastic box where they
"where" is only used when it refers to an actual place/area. In addition, "in which", if appeared in the answer choices, is usually better than "where" when leading a clause.
(C) a disposable plastic box in which to
look at the explanation below.
(D) disposable plastic boxes inside which they
There is no such idiom as "inside which".
(E) the disposable plastic boxes in which to
look at the explanation below.
Remember:
particular reference (the + noun) vs. general reference (plural noun or a/an + singular).
Here, a particular reference should be used because "a replacement" is designed for something specific.
C: a disposable plastic box ---> general reference wrong!
D: disposable plastic boxes ---> general reference wrong!
E: the disposable plastic boxes ---> particular reference. also wrong! why???
Even though E has particular reference, it sounds like it is referring to boxes that are produced within a specific timeframe or with a specific lot number etc. And it actually shrinks the range the "box" refers to, whereas in A, "the disposable plastic box" refers to a box that is being used by the entire "compact disc" industry. In the words of mathematics: <the boxes> ⊂ <the box>.
It might be hard to understand, but you can always eliminate these answer by idioms of "which".