Winners of the crafts prize, three who are ceramists, will display the

Here the underlined part just refer to winner of crafts prize, the correct idiom would be "three of whom".

"Three of who" is used to refer to him/her i.e for direct pointing to specific person.
So,option is close but not correct just a subtle meaning away.

Other Options uses incorrect idiom to refer to people in this case.Those are normally used to refer to things/objects.

So, Option D is correct.

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Winners of the crafts prize, three who are ceramists, will display their work to the general public in a forum that will include demonstrations of crafting techniques.

A. three who---> changes the meaning of the sentence
B. three of which-----> which cannot refer to people
C. three that-----> changes the meaning of the sentence
D. three of whom----> correct answer
E. three which----->doesn't make sense at all

Official Explanation:

Since the underlined portion of the sentence contains the phrase three who, check whether this phrase is used correctly. In this case, the phrase three who is idiomatically incorrect because the preposition of is required to create the correct idiomatic structure, three of whom. This is an idiom error, so eliminate choice A and look for any obvious repeaters. Choices C and E are obvious repeaters because they also omit the preposition of, so eliminate choices C and E. Now, evaluate the remaining answer choices individually, looking for reasons to eliminate each.

Choice B uses the construction of which. The use of the pronoun which is incorrect since who or whom is correct to refer to winners, so eliminate choice B. Choice D uses the correct idiom construction of whom and introduces no new errors, so keep choice D.

Choice A: No. The idiom of whom is incorrectly written as who. Idiom.

Choice B: No. This choice incorrectly uses the pronoun which to refer to people. Pronoun Agreement.

Choice C: No. The idiom of whom is incorrectly written as that. Idiom.

Choice D: Correct

Choice E: No. The idiom of whom is incorrectly written as which. Idiom.

The correct answer is choice D.

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