In this question, we have the following reaction:
4 NH3 + 5 O2 -> 4 NO + 6 H2O
We have:
1.20 moles of NH3 at STP
In this case, we need to think about molar ratios, for example, between NH3 and NO, the molar ratio is 4:4, which is basically the same number of moles that reacts, will be the same number of moles of NO that will be produced, therefore we will also have 1.20 moles of NO being produced
For H2O, the molar ratio is 4:6, so if we have 1.20 moles, we will have:
4 NH3 = 6 H2O
1.20 NH3 = x H2O
x = 1.8 moles of H2O
Now if we add the number of moles of NO and H2O, 1.20 + 1.80 = 3.0 moles, we will have a total of 3.0 moles of products