which are valid#c variable name?

Answer 1

The answer is in the course.
Go to Basic concepts> Variables > first lesson

Answer 2

No spaces.
No numbers.
You can not use already used names, example: if, for, string, class.
You may use _ , example: string _name = "Andre";

And remember that C# is case sensitive.
"string name" and "string Name" are two completly different names.

Answer 3

Numbers can be used as long as the first character is not a number.

Answer 4

variables which start from "letters" or " underscore (_) "