Answer:
When someone says, "He's giving you a party line," it suggests that the individual is expressing something everyone in their organisation says, such as a rule or something everyone must believe. They appear to be imitating what their group says rather than coming up with their thoughts. It's similar to when you're playing a game with your pals and they all agree on something, so you say it as well, even though you're not sure if it's true or not. It is not being honest and articulating what you truly believe but just following what the group says.