The highlighted portion in option E for parallelism.

E is the answer.

Question Explanation

The underlined portion of the sentence contains part of a list, so check that all items in the list are parallel. The list begins with the infinitive to reduce in the non-underlined portion. The list continues with the verb decrease and the infinitive to avoid. The preposition to appears before the first and third verbs in the list, but not the second, so this is a parallel construction error. Eliminate choice A and look for any obvious repeaters. Because there are no obvious repeaters, evaluate the remaining answer choices individually, looking for reasons to eliminate each.

Choice B contains the list to reduce, decrease, and avoiding. Because these verb forms are not parallel, this is a parallel construction error. Eliminate choice B. Choice C contains the list to reduce, to decrease, and avoiding. Because these verb forms are not parallel, this is a parallel construction error. Eliminate choice C. Choice D contains the list to reduce, to decrease, and avoiding. Because these verb forms are not parallel, this is a parallel construction error. Eliminate choice D. Choice E fixes the original parallel construction error by changing the list to the parallel verbs to reduce, to decrease, and to avoid. Choice E introduces no new errors, so keep choice E.

Alternatively, if it is difficult to spot which grammar rule the sentence is testing, another strategy is to try to identify an error from the answers by looking for either a 2/3 split or differences among the answers. Since three answer choices use the infinitive to decrease and the others use only the verb decrease, this difference is an indication to look for parallel construction errors.

Choice A: No. The verb forms to reduce, decrease, and to avoid are not parallel. Parallel construction.

Choice B: No. The verb forms to reduce, decrease, and avoiding are not parallel. Parallel construction.

Choice C: No. The verb forms to reduce, to decrease, and avoiding are not parallel. Parallel construction.

Choice D: No. The verb forms to reduce, to decrease, and avoiding are not parallel. Parallel construction.

Choice E: Correct.

The correct answer is choice E.
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Here was my line of reasoning that got me the correct answer choice, feedback is much appreciated:

The verb requires takes either the command subjunctive or the infinitive. So all verbs after require must take one of these two forms.

answer choice E is the only one that makes sense given this grammar rule.