Economic models:

A. Were once thought to be useful, but that is no longer true

B. Cannot be useful if they are based on false assumptions

C. Can be useful, even if they are not particularly realistic

D. Must incorporate all aspects of the economy if they are to be useful

The correct answer is letter "C": Can be useful, even if they are not particularly realistic.

Explanation:

Economic models are real-world phenomena simplified assumptions that allow individuals to have a better idea of how and why economic events take place and provide certain information on how to mitigate their impact if negative.

Those assumptions are subjective since are given by individuals based on their beliefs but most of the time after observing different events and classifying them using different criteria. Thus, the assumptions might not be accurate sometimes but the economic model concluded could be useful still.