Answer:
C. Merchants helped to spread Islamic beliefs to other cultures.
Explanation:
The BEST that explains how trade was instrumental in the expansion of the Muslim Empire is:
"Merchants helped to spread Islamic beliefs to other cultures."
Trade was instrumental to the spread of Islam because merchants who traded in other cultures and territories carried Islamic ideas. Such exchanges that took place affected native populations and led to more conversion of Islam. As the Islamic ideas traveled in those trade routes, they mixed with the local cultures.