The acceleration of the object at the highest point=g=-9.8 m/s²
When an object is thrown upwards , its velocity decreases continuously until it reaches the highest point. At the highest point, velocity of the object is zero. The acceleration during the upward journey and at the top is constant and is called as acceleration due to gravity denoted by g. It acts downwards and its value is -9.8 m/s².
So the acceleration at the top =-9.8 m/s²