Step 1: Check the Reason first, since it is a simple structural fact.
The carboxylic carbon is $sp^2$ hybridized, and all three groups attached to it, the carbonyl oxygen, the hydroxyl oxygen, and the carbon chain, lie in one plane, exactly like the geometry around a typical carbonyl carbon too. So Reason (R) is a correct statement about the shape of the carboxyl group.
Step 2: Now examine the electron density on the carboxylic carbon.
In a plain carbonyl group like an aldehyde or ketone, the carbon has only the C=O double bond pulling electron density away, leaving a strong partial positive charge and high electrophilicity.
Step 3: Bring in the effect of the extra $-OH$ group in a carboxylic acid.
In $-COOH$, the lone pair on the hydroxyl oxygen can be pushed into the ring system by resonance, donating electron density back onto the carbonyl carbon. This resonance donation cancels out part of the positive character that the C=O group would otherwise create.
Step 4: Compare the two electrophilicities.
Because of this extra electron donation, the carboxylic carbon actually carries less positive character than a simple carbonyl carbon, making it less electrophilic, not more. So Assertion (A) is false.
Step 5: Check the link between A and R.
Since A itself is wrong, the question of whether R "explains" A does not even arise here; we only need to confirm R stands as an independent, correct fact, which it does.
Step 6: Final answer.
\[ \boxed{\text{(A) is wrong, (R) is correct}} \]