The aim of the new regulations is to make it easier for prospective ho

Hello, buan15. It in choice (A) is what is known as a placeholder or placeholder noun. As such, it serves no semantic purpose—i.e. there is no antecedent—but the word provides a necessary subject for the construct used in the latter part of the sentence. If you look at the second definition of placeholder on the Oxford Learner's Dictionaries site, here, you will see an example with it serving the same purpose. In English, it is common to see such usage, and there are official questions that adopt the same placeholder in the OA.

If you have further, questions, let me know. Good luck with your studies.

- Andrew

Why the choice D The aim that is wrong?

This sentence is about "the aim (= goal/purpose) of" a certain set of rules. This aim/goal is to CHANGE the current situation (into a new situation in which it's easier for certain people to do something).

Choice A:
The goal/aim is to make it easier for prospective homeowners to have funds...
"To MAKE it easier" properly expresses the idea of changing things so that this becomes true.

Choice D:
The goal/aim is that prospective homeowners more easily can have funds...
"That THEY CAN..." says nothing about accomplishing a change. This phrasing illogically indicates that prospective homeowners already "CAN" easily have these funds available now—which not only yields a sentence that doesn't specify anything that could reasonably be a goal or statement of purpose, but also makes it unclear exactly what comparison is made by "easier" (easier than when / for whom / in what other situation?).