Part of the given sentence is underlined. Choose the answer choice which is…

Concept

In a 'choose the best version of the underlined part' item, the correct choice is not simply whichever wording is grammatically parsable -- it is whichever is the most concise, direct, and idiomatic while keeping the two verb phrases joined by 'and' in parallel form (same tense, same voice, same directness). A wordier or more indirect paraphrase can still be technically parsable and yet be a weaker 'version' than a short, direct, tightly parallel one.

Application

Here the subject 'Drinking milk enriched with Vitamin D' takes the modal 'may', governing two phrases joined by 'and': a risk-reduction phrase and an aid phrase. The most direct and parallel way to phrase both is with short, active verbs taking their objects straight, with no preposition and no extra circumlocution: 'reduce the risk of rickets' and 'aid the sufferers of heart disease'. Reading the sentence with 'significantly reduce the risk of rickets and aid the' inserted gives exactly this -- two short, parallel, active-verb phrases, with 'aid' taking its object 'the sufferers' directly and no preposition.

Cross-check / Contrast

• Keeping 'aid for' (the original wording) leaves the verb 'aid' followed by the preposition 'for'; idiomatic English says 'aid someone', not 'aid for someone', so this is a genuine idiom error, not just a style choice.

• Replacing 'significantly reduce' with 'be significant in reducing' stays grammatically parsable, but it swaps a short, direct verb for a wordier, indirect construction, making the phrase noticeably less concise and less tightly parallel with the plain-verb clause that follows.

• Replacing 'significantly reduce' with 'cause a significant reduction in' also stays parsable (reading 'aid to sufferers' as a noun phrase), but it swaps a short, direct verb for a heavier noun-based construction, again making the phrase wordier and less tightly parallel than the plain-verb version.

The choice pairing the short, direct, active verb 'reduce the risk of rickets' with the equally direct 'aid the sufferers' is the most concise, idiomatic, and tightly parallel version of the underlined part among the four -- which is why it is the best answer.