Money serves as a ....71..... of exchange modern world.

Analyze the question: The sentence is "Money serves as a ____ of exchange in the modern world." We need the word that correctly completes the common phrase about the role of money.

Explain the concept: In economics, money has several functions. One is to act as a medium of exchange, meaning it is the intermediary people use to buy and sell goods and services instead of bartering. "Medium of exchange" is a fixed collocation (a set phrase), so the other words don't fit naturally.

Solution, step by step:

1. Value: "value of exchange" is not a standard phrase. Money does serve as a store of value, but that is a different function.
2. Medium: "medium of exchange" is the correct, standard phrase. ✔
3. Type: "type of exchange" would mean a kind of exchange (e.g. barter), not what money is.
4. Form: "form of exchange" is understandable, but it is not the standard term. "Form of payment" or "form of currency" would be more natural.

Answer: The correct option is B. medium.

Tip: In WASSCE lexis and structure questions, look for fixed collocations. Phrases like medium of exchange, store of value, and unit of account are favourites from Economics, so learning them helps in English and Economics papers.