Concept:The correct prepositional phrase expresses an exchange relationship: giving one item in return for another item.
Explanation:The sentence means that in former times, cowries were given as a form of payment in return for food.
The phrase "in exchange for" is the standard idiomatic expression used in English when one thing is traded or swapped for something else.
Option A, "on exchange for," and option D, "on exchange with," are not grammatically acceptable because "on" is not used with this phrase.
Option B, "in exchange of," is wrong because the correct preposition after "exchange" in this idiom is "for," not "of."
Thus the sentence should read: In the olden days, people gave cowries in exchange for food.
Answer:C. in exchange for