Concept:
An antonym is a word that expresses the opposite meaning.
Step 1: Meaning of frequent.
Frequent means:
• Happening often.
• Occurring repeatedly.
• Common.
Step 2: Determine the opposite meaning.
The opposite should mean:
• Uncommon.
• Infrequent.
• Not occurring often.
Step 3: Compare options.
Regular and often are close in meaning to frequent.
Sometimes indicates occasional occurrence but is not the exact opposite.
Rare means uncommon and infrequent.
Step 4: Conclusion.
Hence, the antonym of frequent is
rare.