Can you tell me the simple explanation what the mol in chemistry is?

By the way it's not mol, it's mole.
The mole is the unit of amount in chemistry.  It provides a bridge between the atom and the macroscopic amounts of material that we work with in the laboratory.  It allows the chemist to weigh out amounts of two substances, say iron and sulfur, such that equal numbers of atoms of iron and sulfur are obtained.  A mole of a substance is defined as:The mass of substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of 12C.

The correct answer is b) false