The activation energy for the reaction no2(g)+co(g)⟶no(g)+co2(g) is ea = 375 kj/mol and the change in enthalpy for the reaction is δh = -225 kj/mol . what is the activation energy for the reverse reaction?

Answer is: 600 kJ/mol.
Chemical reaction: NO₂(g) + CO(g) ⟶ NO(g) + CO₂(g).

Ea(forward) = 375 kJ/mol.
Ea(reverse) = ? kJ/mol.
ΔH(reaction) = -225 kJ/mol; this is exothermic reaction.
The enthalpy change of reaction is the change in the energy of the reactants to the products.
ΔH(reaction) = Ea(forward) - Ea(reverse).
-225 kJ/mol = 375 kJ/mol - Ea(reverse).
Ea(reverse) = 600 kJ/mol.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.