The correct answer is A.
A gag rule is a regulation that explicitly prevents discussion about a certain issue from taking place in the legislative chambers or other public decision-making bodies.
When the abolitionist movement started to agitate the US politics, claiming against the institution of slavery. For example, John Quincy Adams, as a member of Congress, sent repeated petitions claiming for abolition. The pro-slavery factions established a gag rule which automatically blocked his petitions and these stopped to be discussed or read in the chamber.