Resonating structures of (A) and (B) are as follows
Structure (II) is less stable than structure (I) because later carries separation of positive and negative charges. Therefore, contribution of structure (II) is less than that of (I) towards the resonance hybrid of compound (A), i.e., `CH_(3)COOH`. On contrary, structure (III) and (IV) are of equal energy and hence contribute equally towards the resonance hybrid of compound (B). Therefore, structure (B) is more stable than structure (A) i.e., `CH_(3)COO^(Θ)`.