Fruit type of mustard is

The fruit type of mustard is Siliqua.
Silique is any dry fruit that separates at maturity into two or four segments called valves, leaving a persistent partition that bears the seeds.
It is a type of fruit (seed capsule) having two fused carpels with the length being more than three times the width. When the length is less than three times the width of the dried fruit it is referred to as a silicle.
The valves remain connected at the top. A typical silique is an elongated capsule, such as in cabbage.