Is it not possible that v=0 then accelaration is also zero if a particle is thrown upward then at highest point at v=0 but a not equal to zero?
At the highest point, particle’s velocity is 0 but its acceleration is g. That’s because just before or just after that instant the velocity is not 0 and so its acceleration is nonzero.
Acceleration is the rate of change of velocity. That means it is the difference of final and initial velocity divided by the time duration between these two observations. Obviously, that means that there must be two points of time within which acceleration happens. You take any two instances of time and get the instantaneous velocities at these two instances and divide that by the interval of time, you are bound to get 9.8 m/s^2.