The sudden appearance of the stranger in the cottage despite all doors being closed is questionable.

A. casual

B. surprising

C. dubious

D. certain

Answer: Option D

Solution(By Examveda Team)

Questionable : doubtful as regards truth or validity.
Casual : relaxed and unconcerned.
Surprising : causing surprise; unexpected.
Dubious : hesitating or doubting.
Certain : able to be firmly relied on to happen or be the case.

Antonym of Questionable is Certain