Native English speakers only, who or which?

Who because Tom and Mary are people. Which is only used if what you are referring to are objects or things.

B. is wrong because it must go in front of a noun. You could say:

Which person is smarter, but not just "which is smarter".

Or you could say: Which one is smarter. Sure, people

will still understand you, but it's not grammatically correct.

It is A because which B refers to a thing and not a person

WHO is smarter, Tom or Mary ?

WHO is used here because you are being asked to choose between Tom and Mary as "named people".

However, it would be ..

WHICH person is smarter, Tom or Mary ?

because the choice here is now between two ''people'' (who are only named later on)

The question is almost refering to Tom and Mary as two ''objects'' to choose between.

WHAT is something else altogether.

It is asking for an opinion, so in Tom and Mary's case it could be used like this .

WHAT do you think, is Tom smarter than Mary ?

A. Who

Tom and Mary are people.

A. Who

"Who" is used when you are speaking about people/human beings. Ex: who is that person?, who is the president of usa?,etc.

"Which" is used to refer to things/objects. Ex: Which number is greater? Which way should i go?,etc.

A. WHO

"Who" is used with people, as in your question of "Who is smarter, Tom or Mary?"

"Which" is used with things

"Which book did you read?

A. Who