Concept:The verb "prefer" uses a fixed preposition pattern when comparing two things.
Explanation:"Prefer" means to like one thing more than another.
For comparisons, the correct structure is "prefer + noun + to + noun".
Thus, the sentence must be "I prefer oranges to mangoes".
Option A "on" is incorrect because "prefer" is never followed by "on" for comparisons.
Option B "than" is used after comparative adjectives, such as "better than", but not after "prefer".
Option D "for" is not grammatically acceptable in this pattern.
Therefore, option C "to" correctly completes the sentence.
Answer:C. to