Answer:
The correct answer would be option D, Focus their thinking on the essence of the problem at hand.
Explanation:
An economist is a person who has expertise or who is an expert in the field of Economics. The Economists make assumptions about the economies to focus their thinking on the essence of the problem they have at the moment. They predict the economy, they analyze the economy, they suggest solutions to the problems concerning the economy on the basis of their experience and the new studies being carried up by them. So for this purpose they make assumptions because they want to clearly analyze the situations and want to see what impact their decision will make on the economy, which give them a clear picture about the possible solutions of the problem they have at hand at the moment.