Assertion: The specific charge for positive rays is a characteristic constant.
Reason: The specific charge depends on charge and mass of positive ions present in positive rays.
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. Both assertion and reason are incorrect.

Hint: We have to overlook the concept of specific charges if they are the same or different for different gases. Second, we have to consider the mass and how they affect the particular gases.

Complete step by step solution:
When we talk about different gases it is important to understand that every gas is different in all aspects. But if we look into the specific charges for the positive ion as said in the assertion then the specific charge is fixed for particular gases. That means it is different for different gases. Therefore, the assertion that the specific charge for positive rays is a characteristic constant is correct.

Now, let us see reason, reason specifies that the specific charge depends on charge and mass which is correct but it does not explain the assertion. That means the assertion is correct but the reason for this assertion is not the correct explanation.

Therefore, the correct answer is option B.

Note: Whenever we need to solve such a question we have to think about assertion first and then about the reason because reason is always considered as explanation for assertion and if it is correct we will say reason is the correct explanation if not we will say reason is not correct explanation.