Number of atoms

$24 \;g$ of carbon $=2 \times 6.023 \times 10^{23}$

$56 \;g$ of iron $=1 \times 6.023 \times 10^{23}$

$27 \;g$ of aluminium $=1 \times 6.023 \times 10^{23}$

$108 \;g$ of silver $=1 \times 6.023 \times 10^{23}$

Hence the maximum number of atoms is in 24 g of carbon (ie 2 moles of carbon)

Hence a is the correct answer.