A ball is thrown vertically upwards. What is its momentum at the highest point?

Momentum is the product of mass and velocity. As the velocity at the highest point is zero because when the ball moves upwards, its velocity is opposite to the gravitational force which deaccelerates the ball and velocity shifts to zero and stops just at that point (for a transient period) and further due to earth's gravity, the ball falls.

Now as the velocity is zero at the highest point.

Momentum = Mass × Velocity

p = m × v

p = m × 0

p = 0

Hence, momentum is zero.