One of the principles of standard English usage is that if a sentence presents multiple related items or phrases, then each of those items or phrases should be presented in parallel grammatical structures. Parallelism is also called parallel structure or parallel construction.
Parallelism is one of the favourites of GMAT. It involves grammar as well as logic. Suppose A and B are two phrases or clauses that you want to put in parallel. There are two things to consider - both A & B should be grammatically correct and both must match grammatical form.
The basic rule of parallel structure is that all phrases joined by conjunctions should be constructed in the same manner.
Single words, multi-word phrases and clauses can be made parallel by ensuring that each item is equal. Examples of parallel structure include multiple gerunds or -ing verbals that function as nouns, past participles, infinitive phrases (to be) and complete clauses that include a subject and predicate. Never mix and match when using parallel structure to create a list or series.
The problem here is that the first two items in the list are nouns (loss and decrease), but the third item is a gerund (destroying). The parallel form would be destruction.
Two parallel elements (phrases or clauses) are joined by conjunctions (coordinating or correlative). The most common types are:
The consultant recommended that the company eliminate unneeded positions, existing departments should be consolidated, and use outsourcing when possible.
The underlined portion of this sentence contains a list joined by and, which is a pretty good clue that you should be vigilant for any lack of parallelism. Because the three phrases joined by and are not all constructed the same way, you know there is an error, so eliminate Choice (A).
Next, eliminate the answers that don't solve the problem. Choice (C) keeps the same faulty construction as the original statement in the first two recommendations, and it introduces even more awkwardness by changing use to used and adding it to the end of the third recommendation. You can clearly eliminate Choice (C).
Eliminate Choice (E) because it's also worse than the original. Each of the three elements in Choice (E) has a completely different construction.
Both Choice (B) and Choice (D) seem to correct the error by introducing each recommendation with a similar construction, but Choice (D) creates a new error because it changes the meaning of the sentence.
Choice (B) solves the problem without changing the original meaning, so it is the correct option.