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