Question

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

Answer 1

zero because at the highest point it's velocity becomes zero
and momentum = mass × velocity
so when velocity becomes zero then momentum will also becomes zero

Answer 2

it's highest acceleration would be 9.8m/s^2 and if thrown perfectly vertically it's velocity at the highest point will be 0 therefore it's momentum would be it's mass multiplied by velocity.
therefore : momentum= mass * velocity
let mass be m
than
momentum= m *0
momentum= 0