Number of atoms in 4.25 g of NH3is approximatly............

 Molar mass NH3 = 14+3 = 17g/mol 

given mass = 4.25g

no of moles = 4.25/17 = 0.25 mol of NH3 

1 mol of NH3 contains 6.022*10^23 molecules 
0.25 mol NH3 contains 0.25*6.022*10^23 = 1.5*10^23 molecules 

1 molecule of NH3 contains 4 atoms 
Number of atoms in 1.5*10^23 molecules =  4*1.5*10^23 atoms. 
= 6*10^23 atoms 
 

The correct answer is = 6*10^23 atoms.