D. Watching television makes people more intelligent.
The author provides two examples to back up that watching television makes people more intelligent: First, they mention that people who watch television are more informed about the world (They are more aware, and can have a better view on what's going on), and second, they mention that people who regularly watch the World History Channel answer questions correctly 10% more often than those who don't watch the channel, meaning that these people are better at test than others who do not watch T.V.
The other options are incorrect:
A. Watching television is a complete waste of time: The author claims exactly the opposite.
B. People are aware that television is a good source of information: The author begins by asserting the opposite "Many people believe that television makes a person less intelligent".
C. People who watch more television are better at tests. Although the author does make this point, it is just a part of his main claim which is that people who watch more television are intelligent overall.