Answer: A. encourage the group to establish its mission and purpose.
Step-by-step explanation:
A Team leader helps their group by giving them direction. By giving them a goal. There is no better way to do this than by helping them establish it's MISSION and PURPOSE.
Once these are established, a group can have a Direction.
This is what a leader does. They galvanise their members into a coherant unit that can carry out tasks but first, they need to know what kind of tasks to be engaged in.
Think about how Captain America takes charge of the Avengers along with Tony Stark and tells them what needs to be done and how to do it.
That is Leadership.