(A) S_N2 reactions are exothermic
(R) S_N2 reactions are thermochemically favored by stronger nucleophiles

S_N2 reactions (bimolecular nucleophilic substitution reactions) occur in a single step, where the nucleophile attacks the substrate and simultaneously displaces the leaving group. These reactions are often exothermic because the products (typically a stronger bond between the nucleophile and the substrate) are more stable than the reactants. The reaction is thermochemically favored by stronger nucleophiles, which have higher electron density and are more reactive in attacking the substrate.

Explanation:
(A) S_N2 reactions are exothermic: This statement is true because the formation of a stronger bond between the nucleophile and the substrate releases energy, making the reaction exothermic in most cases. 
(R) S_N2 reactions are thermochemically favored by stronger nucleophiles: This statement is also true because stronger nucleophiles are more reactive and result in faster and more efficient substitution reactions. However, the exothermic nature of S_N2 reactions is due to bond formation and not solely because of the nucleophile's strength.

Since both (A) and (R) are correct, but (R) does not directly explain why S_N2 reactions are exothermic, the correct option is: 
b) Both (A) and (R) are correct, but (R) is not the correct explanation of (A).