An organization that seeks to gain power through legitimate means.

The correct answer is: "a political party".

A political party is an organization that aims to gain legitimacy to govern a country, state or municipality by obtaining the largest share of votes from the citizens when elections are celebrated.

The power of a country resides on its citizens, who can grant it to political representatives through suffrage. Such representatives are, in turn, organized in political parties that, when they win an election, will have the chance to constitute a goverment and represent their interests.