The acceleration of an object refers to the change of velocity at a certain rate including both slowing down and speeding up.
(i) So it is possible for the object to be slowing down while its acceleration is increasing in magnitude. In such a case, the object suffers an extreme deceleration and comes to rest rapidly.
(ii) It is also possible for the object to be speeding up while its acceleration is decreasing. In such a case, the acceleration goes on decreasing to become zero and finally the...

Since the horizontal component of the velocity u of such a projectile is constant, there is no acceleration in this direction. So the acceleration of the projectile is taking place in the vertical direction. The acceleration is parallel or anti-parallel to the vertical component of u and hence there is no point at which the acceleration is parallel to u. At the maximum height, the vertical component of u is zero but u is horizontal. So the acceleration is perpendicular to u at the maximum...