Which of the following is the smallest order of class-Insecta?

Answer: Option B

Zoraptera is the smallest order of class-Insecta.
The insect order Zoraptera, commonly known as angel insects, contains small and soft bodied insects with two forms: winged with wings sheddable as in termites, dark and with eyes (compound) and ocelli (simple); or wingless, pale and without eyes or ocelli.
They have a characteristic nine-segmented beaded (moniliform) antenna. The members of this order are small insects, 3 millimeters (0.12 in) or less in length, that resemble termites in appearance and their gregarious behavior. They are short and swollen in appearance.