A. Most companies that require their employees to sign such agreements are aware that these documents are not legally binding
We don't know whether employers are aware of such laws. They may very well be aware or not. This doesn't explain why competitors refrain from hiring them
B. In some industries for example, the broadcast media, companies' main source of new employees tends to be people who are already employed by competing firms
This doesn't explain anything at all
C. Many companies will not risk having to become involved in lawsuits, even suits that they expect to have a favorable outcome.
correct competitors don't want to be involved in any lawsuits so no matter what they stay away from hiring for a period of time
D. Many companies consider their employees established relationships with clients and other people outside the company to be valuable company assets
This doesn't explain anything at all
E. Many people who have signed such agreements are unwilling to renege on a promise by going to work for a competing firm
This is a trap. We are given that companies refrain from hiring. It doesn't matter whether people want to change their jobs. In the end they can still be approached by such firms wanting to hire