7)an object is thrown straight up with an initial velocity of 3 m/s. what is its acceleration at its maximum height? a)zero b)3 m/s2 c)-9.8 m/s2 d)none of the above

C.
It is a motion with uniform acceleration, meaning that the acceleration will not change.
The object is thrown upwards with a positive velocity. This shows that the upward direction is positive. The object will decelerate due to gravity at a magnitude of 9.81 m/s2. Therefore, the acceleration is -9.81 m/s2.
Note that even though the velocity of the object is momentarily 0 m/s at maximum height, there is still a constant acceleration.
This allows the object first decelerate upwards, then change direction at max height, and finally accelerate downwards. So in this case, the acceleration is always negative and unchanged.