The answer here is:
B. A radio producer chooses to drop a report about child obesity to make time for a report about new immigration legislation.
Here's why...
Gatekeeping can be defined as deciding which information is allowed to be disseminated or broadcast to the public at any time. In this case, the radio producer is deciding that immigration legislation is more important than child obesity.
Some people may disagree with the opinion of the producer and so this can be seen as a selection based on his or her own opinions of the news and current affairs.
You could also argue that the blogger is a gatekeeper, but I would say that the strongest example here is the radio producer as radio can be identified as mass media which is more accessible for most people.