Present a threat to mainstream media, largely because they

Blogs; are cheap to produce.

Explanation:

Blogs are a dangerous alternative to mainstream media. In a blog, the author performs many of the same actions that a large team at a media company produce, such as researching, writing, recording, editing, publishing, advertising, etc. Moreover, blogs are usually very cheap to produce, and as they require very little investment, can often be very profitable.

i am pretty sure it is d.