Finding the energy given out when 2 moles of reactants are mixed and ignited from a balanced equation
The N2O4 is going to define the maximum amount that will be used up in the reaction, since it has the higher stoichiometric number of the reagents. That is, the second term will always be less and the amount consumed will be defined by the N2O4 because that two moles will limit the N2O4 and in doing so reduce the amount of the second term consumed- there will be excess second compound after the N2O4 has been consumed.

For example, the ratio of first to second term is 5:4, so if we consume all of the second term (2 mols) this requires 2.5 mols of N2O4, which we don't have. So we are limited with the N2O4.

So, knowing that, with two mols of N2O4 the amount is reduced to 2/5 of the enthalpic calculation. That becomes 5116*2/5 = 2046, or answer B.

Apologies for awkwardness of this answer, on a mobile device and have to keep swapping from question to answer.