He is fond __________ music.

The correct answer is B: of. The phrase "fond of" is a standard and widely used idiom in English, signifying a strong liking or affection for something or someone. Prepositions play a vital role in linking words within a sentence, establishing relationships, and ensuring clarity of meaning. In this specific context, "fond" functions as an adjective that inherently demands the preposition "of" to properly convey its meaning when indicating the object of one's affection. Using alternative prepositions such as "on," "in," or "at" would result in grammatically incorrect phrases or significantly alter the intended meaning. For example, expressions like "fond on" or "fond in" are not recognized as correct English constructions. Mastering these fixed prepositional phrases, like "fond of," is fundamental for achieving accurate, natural, and fluent communication in English. Many English learners frequently make mistakes with prepositions, highlighting the importance of learning and memorizing these idiomatic expressions to enhance both grammatical precision and overall fluency.