The best answer to the query "Why is it often easier to start a fitness program than it is to maintain one?" is A. The motivation at the start of a program is often higher than later on when the progress made might be less obvious.
In summary, while all options touch on different aspects of starting and maintaining a fitness program, motivation (option A) is the primary and most relevant factor that commonly explains why many people find it easier to begin than to continue their fitness journey.