Explanation:

The meaning of the word 'CHRONIC' is  persisting for a long time or constantly recurring.

In order to find the correct antonym for the given word we must see the options and their meanings.

1. routine - a sequence of actions regularly followed

2. regular - arranged in or constituting a constant or definite pattern, especially with the same space between individual instances.

3. infrequent - not occurring often; rare.

4. continuing - without a break in continuity; ongoing.

Hence after looking at all the words and their meanings it is clear that 'infrequent' is the correct antonym for the word chronic. So option 3 is the correct answer.