Which one of the following has maximum number of atoms? 

Number of atoms $=\frac{\text{ Mass }}{\text{ GAW }}\times N_A$
When mass is same, number of atoms is inversely proportional to GAW.
Atomic weight N_A of atoms
$1\mathrm{g}\mid \text{ of }\mathrm{Li}\to \frac{{\mathrm{N}}_{\mathrm{A}}}{7}\text{ atoms }$
(1) 1 g of Ag $\to \frac{{\mathrm{N}}_{\mathrm{A}}}{108}\text{ atoms }$
(2) 1 g of Mg $\to \frac{{\mathrm{N}}_{\mathrm{Al}}}{24}\text{ atoms }$
(3) 1 g ${\mathrm{O}}_2\to \frac{{\mathrm{N}}_{\mathrm{A}}}{16}\text{ atoms }$
Hence, 1 g of Li has maximum number of atoms.