This question is testing PRONOUNS and MEANING.

Since 1992, in an attempt to build up the Atlantic salmon population in each of the seven rivers in which salmon still spawn, state officials in Maine have stocked them with fry raised in hatcheries from eggs produced by wild fish found in that particular river.

(A) them
- there is pronoun ambiguity. Because "them" could refer to "state officials" or "rivers" (or possibly even "salmon"), we should use a noun rather than a pronoun.

(B) the river
- since we're talking about seven rivers, it wouldn't make sense for them to just stock 1 river.

(C) the rivers
- this seems to work... until we get to the phrase "that particular river." The antecedent for "that" must clearly be singular.

(D) each river
- this works! It's a grammatically singular term, so we can use "that river" to replace it. But it also tells us that we're stocking all 7 rivers.

(E) that river
- "that" has to refer back to something previously mentioned. There was no 1 particular river mentioned earlier - in fact, this would have the same singular river problem as B.

The only logical answer is D.