Why would lobbyists need regulation?
a. to control the pressure applied to politicians c. to ensure that lobbyists will be allowed to continue in politics b. to control the number of lobbyists d. to give an outlet for grassroots groups where they can gain power

The best answer is A. The problem is that when lobbyists are funded by extremely wealthy, powerful interests (like giant corporations or industries), they can have a huge influence on government (in other words, they can exert huge pressure on politicians). It's very difficult for poorer, less powerful people to compete with that kind of power, which gives them less say in how their government works. That's not really an ideal form of democracy, which is why lobbyists need to be regulated—to be sure that everyone has a fair chance to petition the government.