Assertion (A) It is impossible to determine the exact position and exact momentum of an electron simultaneously is the statement of Heisenberg’s uncertainty principle which is mathematically expressed as :

And that is why a clear definite path or trajectory of an electron cannot be predicted accurately and therefore gives rise to the concept of probability of finding the electron in a certain region (probability density).

Hence, Reason (R): The path of an electron in an atom is clearly defined. Is a false statement and therefore cannot justify (A).