He is in the habit of taking a casual leave quite often.

Answer: Option C

Casual : relaxed and unconcerned.
Futile : incapable of producing any useful result; pointless.
Regular : arranged in or constituting a constant or definite pattern, especially with the same space between individual instances.
Formal : (of a person or their manner) strictly conventional.
Systematic : done or acting according to a fixed plan or system; methodical.

Antonym of Casual is Formal