Answer:
a. The practice of ex-legislators becoming lobbyists.
Step-by-step explanation:
The term "revolving door" is a metaphor that implies the way high ranking government officials are involved as legislators and regulators. This practice led to what is generally known as "lobbyist", which refers to those people who are hired by interest groups to try to influence government policies.
This practice of "hiring" high profile government employees for private gains and work is known as the practice of lobbying. This allows many decisions to be influenced in such a way that a particular group gains from it and influences the decision-making process of even high government institutions such as Congress. Therefore, the term "revolving door" refers to the practice of ex-legislators becoming regulators, involved in lobbying for a particular side/party.