Step 1:
$N$ depends upon the basis of the atomic weight scale.
$12g$ carbon=$6.023\times 10^{23}$atoms
$1g$ carbon=$\large\frac{6.023\times 10^{23}}{12}$
$6g$ carbon=$\large\frac{6.023\times 10^{23}}{12}$$\times 6 =\large\frac{1}{2} N_A$
We know, $1amu=\large\frac{1}{N}$
$= \large\frac{2}{6.023\times 10^{23}}$
$= 3.3\times 10^{-24}g$
Step 2:
Let, molar mass of an element be M amu
Then, $M\;amu=M\times 3.3\times 10^{-24}g$
$\therefore$ mass of 1 mole =$M\times 3.3\times 10^{-24}$
Av.No=$M\times 3.3\times 10^{-24}\times \large\frac{1}{2}$$\times 6.023\times 10^{23} = Mg$
Hence (c) is the correct answer.