Letter A is the answer here is my solution
For N: 7g N x (1 mol N/14g N) = 0.5 mol N
For O: 4g O x (1 mol O/16g O) = 0.25 mol O
get the smallest number and get the ratio
For N: (0.5 mol/0.25 mol) = 2
For O: (0.25 mol/ 0.25 mol) = 1
So the empirical formula will be N2O