Between "who" and "whom": "WHO" is the subject and can be replaced with "he, they" while "WHOM" is the object can be replaced with "him, them". So, our subject "three", we need an object to show that out of all winner, three of them are ceramists.
Since the underlined phrase "three who" of the sentence checks whether the phrase is used correctly. In this case, the phrase ''three'', which is idiomatically incorrect because the preposition of is required to create the correct idiomatic structure, three of whom. This is considered idiomatic error, so therefore, eliminate option A and look for any obvious repeaters. Option C and E are obvious repeaters because they also omit the preposition "of". So also they stand elimination.
Now, evaluating the two options left; for option B, the usage of the preposition "OF" with "WHICH" in this contest is wrong. "OF" should be used with WHO/WHOM. Therefore, option D leave us with no choice than the correct option.
OPTION A - INCORRECT:- The idiom is incorrectly written as WHO
OPTION B - INCORRECT:- This option incorrectly uses the pronoun "which" to refer to people. Pronoun Agreement
OPTION C - INCORRECT:- The idiom of whom is incorrectly written as that.
OPTION D - CORRECT:- Correct choice as it validate the explanation above
OPTION E - INCORRECT:- The idiom of 'whom' is incorrectly written as which
