Interest groups are also called pressure groups mainly because
a. they seek to put pressure on the government to affect policies.
b. they put pressure on individuals to join in the group's interests.
c. belonging to a group can create pressure, or stress, in one's life.
d. they put pressure on group members to act in certain ways.

The answer is a.  They are groups who represent certain interest or concerns in society.  This can range from education, crime, health, trade etc.  They want to government to prioritize their concerns and interests.  They do either through meeting with government representatives or some cases through mass actions.  Government recognizes the impact that interest groups have in politics.  If government does nothing to address their concerns, would either pressure them or simply withdraw their support.  These people will then campaign for opponents of present officials during election period and get them elected since they can best serve their interest.