Step 1: Decode the relationship in the stem.
\(INSIPID\) means tasteless, dull, or \emph{bland}.
\(PIQUANT\) means pleasantly sharp, stimulating, or \emph{spicy}.
Thus the pair shows an antonym relation: bland \(\leftrightarrow\) spicy.
Step 2: Test each option for the same antonym relation \emph{and order.}
(a) \emph{relish : spice} — near synonyms (both relate to flavor), not antonyms.
(b) \emph{tasty : bland} — antonyms, but the order is \textit{tasty} then \textit{bland}, which is the \emph{reverse} of the stem (bland \(\rightarrow\) spicy).
(c) \emph{sweet : dessert} — association, not antonyms.
(d) \emph{flat : spicy} — \textit{flat} \(\approx\) insipid (bland), and \textit{spicy} \(\approx\) piquant; antonyms in the \emph{same order} as the stem.
Step 3: Conclude.
The only pair matching both the \emph{relation} (antonym) and the \emph{order} is \(\boxed{\text{flat : spicy}}\).