Step 1: Read the sentence as a whole.
\textit{“Mango, the most unique fruit, is available in India in plenty.”}
Step 2: Identify the grammatical rule involved.
The adjective “unique” is an absolute adjective.
Absolute adjectives do not take comparative or superlative forms like \emph{more} or \emph{most}.
Step 3: Locate the error.
The phrase “most unique” is grammatically incorrect.
Correct usage would be:
\[
\text{Mango, a unique fruit, is available in India in plenty.}
\]
Step 4: Final conclusion.
The error lies in part \(\boxed{\text{(A)}}\).