Assertion: The specific charge of positive rays is not constant.
Reason: The mass of ions varies with speed.
A. If both assertion and reason are true and reason is the correct explanation of the assertion.
B. If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If the assertion is true but the reason is false.
D. If both the assertion and reason are false.

Hint: In order to solve this question, we will first check the correctness of both the given assertion and reason statements and then we will check whether the reason is the correct explanation of the assertion or not.

Complete step by step solution:
We have given the assertion as the specific charge of positive rays is not constant. Reason: The mass of ions varies with speed, so the specific charge is given by $\dfrac{e}{m}$ and yes the specific charge of positive rays is not constant and the reason is that mass changes with speed due to the relativistic effect as for different particle mass changes with speed hence, the assertion is true and the reason statement is given to us as the mass of ions varies with speed and the reason is also the correct explanation of given assertion as we found that mass varies with speed and due to this specific charges is not constant.

Hence, the correct answer is option A.

Note:It should be remembered that, the relativistic effect on mass of a particle when it moves with high speed is mathematically related as $m = \dfrac{{{m_o}}}{{\sqrt {1 - \dfrac{{{v^2}}}{{{c^2}}}} }}$ where, c is the velocity of the light.