Step 1: Understand the task.
We must find the antonym of 'frequent'. An antonym means the opposite.
Step 2: Find the meaning of 'frequent'.
'Frequent' means happening often, again and again, or commonly.
Step 3: Decide what the opposite should mean.
The opposite of 'often' should mean 'seldom' or 'not often', that is, something uncommon.
Step 4: Test the options.
'Regular' and 'often' are close in meaning to 'frequent', so they are not opposites. 'Sometimes' means now and then, which is only weaker, not the true opposite.
Step 5: Pick the contrasting word.
'Rare' means uncommon and hardly ever happening. This is the clear opposite of 'frequent'.
Step 6: State the conclusion.
The antonym of 'frequent' is 'rare'.
\[ \boxed{\text{rare}} \]