Answer:
40 reams of paper is the best choice.
Explanation:
According to the problem, we need 20,000 sheets of paper. One ream of paper has 500 sheets. So, we can elaborate a relation that expresses this

Where
refers to the number of reams we need to have 20,000 sheets of paper. Solving the equation, we have

Therefore, we need 40 reams of paper to have all 20,000 sheets of paper we need.