The Lewis structures in which there are no formal charges is the most stable. Hence, structure (b) is the more stable form of FNO2.
The question is incomplete but I will try to answer you as much as I can. Now the equation is missing hence we can't numerically caculate the enthalpy chnage of the reaction from bond energies. However, we can do this using the formula; Σbond energy of reactants - bond energy of products.
Concerning the Lewis structures of FNO2, the most stable structure is structure (b) as shown in the image attached where the atoms have no formal charges.
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