Avogadro number is

(a) The number of atoms in  gram Atomic weight of substance

(b) The number of molecules in  gram  molecular weight of substance

(c) The number of atoms in 0.012 kg of C-12

The definition of Avogadro's number of 6.022 × 10²³/mole is the 

number of atoms in gram atomic weight of substance

number of molecules in gram molecular weight of substance

Avogadro's number is formally defined as the number of carbon-12 atoms in 12 grams (0.012 kg) of carbon-12, which is approximately 6.023×10²³.