The correct answer is C) in these districts many groups constituents make up the majority of voters.
Interest groups representing businesses and investors are often among the most successful lobbying groups for all the reasons except "in these districts, many groups constituents make up the majority of voters."
Interest groups play an important role in US politics. These groups are formed by wealthy and influential members that always try to exert some kind of influence in legislators in order to support their interests and agendas. Throughout the years and many elections, they have learned how to talk and influence political candidates and exert pressure to have the kind of legislation that its favorable to them.
The other options of the question were A) such groups have the financial resources to mount sustained campaigns on their own. B) many such groups have been in existence for several decades or more, allowing them to master the legislative system and to develop close ties with legislators. D) These groups constituents contribute heavily to many legislators campaigns and in doing so gain greater access to legislators. E) Economic lobbyists often for obscure or minute changes to tax law about which the public knows little.