It was his practice to go for a walk every morning.

Answer: Option B

Convention : a way in which something is usually done.
Habit : a settled or regular tendency or practice, especially one that is hard to give up.
Fashion : a popular or the latest style of clothing, hair, decoration, or behaviour.
Rule : one of a set of explicit or understood regulations or principles governing conduct or procedure within a particular area of activity.
Practice : the customary, habitual, or expected procedure or way of doing of something.

Synonym of Practice is Habit