What's the meaning of " make the call about" and " putting a stake in the ground"?
The leader has to make the call about what’s reasonable in setting goals—putting a stake in the ground for what the group is going to accomplish and making sure the goals are challenging enough without being impossible.

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Answer

In this context, "call" means the decision.

In common English, we can use physical ideas to explain an abstract concept. "Putting a stake in the ground for..." = establishing. You normally put a stake in the ground when you want to mark boundaries, locations and areas.

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We also use stakes to anchor tents and shelters so in this case, the leader also is setting an anchor point.