Assertion: When a bicycle is in motion, the force of friction exerted by the ground on the two wheels is always in forward direction.
Reason: The frictional force acts only when the bodies are in contact.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Assertion is incorrect but Reason is correct.

Hint:Here in the Assertion it is said that force of friction exerted by the ground on the two wheels is always in forward direction that means there must not be any condition in which force of friction applied in backward direction for the Assertion to be true. Try to find that condition.




Complete answer:
There can be two cases when a bicycle is in motion:
Case1: While riding the bicycle when we applied an external force that is paddling.
Then the external force applied by us will be in backward direction and the force of friction will act in forward direction to the motion of the bicycle.
Case2: While riding the bicycle when we do not paddle or do not apply by external force.
Then there will be motion in the bicycle due to inertia and in this case the force of friction will act in backward direction to the motion of the bicycle.
Therefore, Assertion is False.
Now, we know that frictional force acts only when the two bodies are in contact.
Thus, Reason is true.

Hence, the correct answer is Option D.




Note: Be careful of the extreme words such as always, never, forever etc. while dealing with this type of questions where assertions are given. Because there will always be a possibility that it is a false statement. Whenever such words are used most of the time it is a false statement.