Final answer:
The fact that two blocks move together with the same speed and acceleration when a horizontal force is applied indicates that the friction between the blocks is sufficiently strong to prevent relative motion, confirming the presence of static friction.
Step-by-step explanation:
When a horizontal force is applied to a larger block causing both it and a smaller block sitting on it to move with the same speed and acceleration, it implies that there is a force of friction between the two blocks. This friction, frequently called static friction, acts with a force that is strong enough to prevent the smaller block from sliding relative to the larger block underneath it. Therefore, we can conclude that the frictional force between the blocks must be equal to or greater than the force required to move the smaller block at the same acceleration as the larger one.
If the blocks were to be sliding with different speeds, this would indicate that the frictional force between them was not sufficient to keep them moving together. However, since they are moving at the same speed and acceleration as one unit, the frictional force needs to be just right so as to prevent slipping while not being so large as to cause additional resistance above what is necessary.
Contrary to the suggestion that friction causes the larger block to move faster (answer d from the provided question excerpt is incorrect), friction does not have the ability to increase the speed of an object; it can only oppose motion. The correct understanding of friction is that it acts in the opposite direction to the motion of the moving objects when they slide against each other.