Answer: B. News Coverage
Explanation:
A lot of people watch the news and trust it to give them accurate information about the current affairs of a nation. For this reason it is very important in shaping the public agenda and has a lot of power over public opinion.
News bulletins however, often have a time limit and so cram in as much news as they can which leads to them relying on short sound bites which might not show the full opinion of the presidential candidate. Also as the news is supposed to be independent, candidates have limited control over what is broadcast.