Hydrocarbons, with which fruit flies perfume themselves in species-specific blends, are known to be important in courtship,
and apparently this assists flies that taste the hydrocarbons on prospective mates to distinguish their own species from that of others.
Option Elimination -
(A) and apparently this assists flies that taste the hydrocarbons on prospective mates to distinguish their own species from that of - "this" is a singular demonstrative pronoun that can refer to "courtship" or the concept with which fruit flies perfume themselves. We need a noun after "this." For example, The CEO presented a new business strategy, and this approach received positive feedback from stakeholders. Here, "this" modifies "approach." What approach? "The new business strategy."
Moreover, "assist" is usually used to help someone achieve a goal, to help, support, or aid someone in accomplishing a task. For example, The team members assisted each other in completing the project.
Preferred usages are "assist in" or "assist with."
1. Assist in - when emphasizing the involvement in a broader process. For example, the consultant assisted in developing the company's marketing strategy.
2. Assist with - when indicating helping someone in a specific activity. For example, The teacher assisted the students with their math homework.
Remember, both "assist with" and "assist in" can be used interchangeably in many cases, and the choice depends on the emphasis and context of the sentence.
Additionally, "that" is a singular demonstrative pronoun referring to what? Species or mates? But both are plural. Wrong.
(B) and apparently, this assists flies when they taste the hydrocarbons on prospective mates in distinguishing their own species from those of - The same issue of "this" without a noun after it. "when" is a time marker. So, does it only help if they taste it at a particular moment? That's wrong. "Those" refers to species, but do we need this plural demonstrative pronoun here? "otherS" means other species. So why create redundancies? Wrong.
(C) which apparently assists flies that taste the hydrocarbons on prospective mates in being able to distinguish their own species from. "which" refers to "courtship," creating a wrong meaning. Moreover, "being able to distinguish" is a lousy way of saying "distinguishing." Wrong.
(D) apparently assisting flies to taste the hydrocarbons on prospective mates to distinguish their own species from those of - so here comma + ING becomes adverbial. So "assisting" modifies "Hydrocarbons are known to be." Are we saying "hydrocarbons" are assisting the flies to taste hydrocarbons? Ridiculous. Isnt it? Yes. Moreover, the same "those" redundancy.
(E) apparently assisting flies that taste the hydrocarbons on prospective mates in distinguishing their own species from - ok. So here, "hydrocarbons" are assisting the flies" Which flies? "that taste the hydrocarbons." ok.