You didn't give us answer choices, but we can at least define "rotten boroughs" for you here ... and that may be enough to lead you to the correct answer.
A "rotten borough," also sometimes called a "pocket borough," was a borough or area that had a representative seat in Parliament but no longer really had much population in the borough. People had moved to the cities in droves due to industrialization. But as old parliamentary regions retained their representation in the House of Commons, their small electorate could be controlled by a small faction, or even by a single person. That's where the term "pocket borough" came from, as a whole representative district was essentially "in the pocket" of a single powerful person or family in that district.
Obviously all of that is bad for democracy. Democratic processes work best when the people are represented fairly. How boroughs for Parliament were drawn up, or how congressional districts for the US Congress are drawn up today, has an affect on how people are represented democratically. In the USA today, we have a sort of similar problem in what are called "gerrymandered" districts. Look up that term to learn more!
