The correct option is
B Nut
Fruit of walnut is dry and has a hard outer shell with seed within, it is thus a nut type of fruit.
Drupe is a type of fruit having outer membranous exocarp , fleshy mesocarp and a stony endocarp as is found in peaches.
Follicle is a type of dry dehiscent fruit that spilts only on one side and is found in some flowering plants like larkspur.
Carcerulus is a type of schizocarp fruit not a nut fruit.
So, the correct answer is 'Nut'.