There is nothing wrong with "Please show me what is B."
Let's start with a couple of examples:
Please show me what is right.
The above example is a correct sentence. The expectation is that the speaker will be shown which is the correct, right, proper, preferred, etc. of two or more choices.
Please show me what right is.
The above example is also a correct sentence. But the expectation is that the speaker will be shown something that exemplifies, demonstrates, or defines the meaning of "right" (and in this specific case, we don't know if we're talking about right as in "the right thing to do" or right as in "turn to the right").
However, those two examples do not demonstrate a predictable rule.
A problem is that depending on the context of the two examples, above, the expectations can change.
I understand that you believe you know what is right, but I do not believe you understand what right is. Please show me what is right.
In the above example, the word "right" means correct, proper, preferred, etc., even though both sentence structures were used.
When George Bernard Shaw said, “The British and the Americans are two great peoples divided by a common tongue,” his witty statement failed to reveal the devastating truth. Put two people who believe they understand English in the same room, regardless of education or training, and an argument is inevitable. Such is the curse and the blessing of a living language.