Step 1: Checking Each Pair: ‘Lazily’ and ‘poorly’ are unrelated in meaning; ‘secretly’ and ‘quietly’ are near-synonyms; ‘promptly’ and ‘quickly’ are synonyms.
Step 2: Final Answer: ‘Rapidly’ (fast) and ‘slowly’ (not fast) are opposite in meaning, so (iii) Rapidly — Slowly is the correct antonym pair.