The correct option is B formal
The word 'casual' refers to being relaxed and unconcerned.
Option A, futile refers to being incapable of producing any useful result.
Option B, regular refers to something recurring at uniform intervals.
Option C, formal refers to something officially sanctioned or recognized.
Option D, systematic refers to something done according to a fixed plan or system.
Thus, according to the meaning, the word 'formal' is the opposite of the word 'casual'.