In ancient times, many people believed in a geocentric model of the solar system, where the earth was the center of the system. during the 16th and 17th centuries, men such as copernicus, kepler, and galilei theorized or found evidence that the sun was actually the center of the solar system. however, despite convincing evidence, this idea was not accepted until some time later. why was this idea not initially accepted? a. after a theory has been around for one hundred years, it cannot be changed. b. there was no evidence published supporting their ideas. c. it is difficult to change a commonly held belief. d. the sun is not large enough to be at the center of the solar system.

There is really no best answer to this. So you have to pick the best of  a bad lot. 
If the sun is not large enough the earth isn't either. The church had evidence of what the earth's and sun's radius was. 

Of course there was no evidence published to support their ideas. They were the ones who developed the ideas.

Here's the problem. C and A are saying much the same thing. I would pick C because it's more general. I wouldn't pick A because an Earth centered universe had been around for a lot longer than 100 years. If you agree with me and use that answer and we are both wrong, argue your case and then be quiet. If your marker agrees, well and good. If he doesn't maybe he will next time.