Sporadic

The most appropriate antonym of the word sporadic is continuous.

• Sporadic means "occurring at irregular intervals or only in a few places; scattered or isolated."
• Continuous means "occurring or existing without interruption or cessation."

The other words do not have the same meaning as "sporadic" and are therefore not antonyms.

• Inculcate means "to teach or impress (something) by frequent repetition or instruction."
• Seldom means "not often; rarely."
• Attractive means "having qualities that appeal to the senses or mind."