This question asks you to find a pair that is not a correct synonym match.
Let's evaluate each pair:
(A) Introduction - Preamble: Both relate to beginnings; correctly matched.
(B) Collate - Assemble: Both imply gathering; correctly matched.
(C) Improve - Ameliorate: “Ameliorate” means “to make better”; correct.
(D) Stern - Step:
“Stern” means strict or harsh.
“Step” refers to movement or a stage; unrelated in meaning.
Therefore, (D) is not a synonym pair.