Which insect has evolved resistance to Bt in open field population?

Answer: Option B

Diamond back moth insect has evolved resistance to Bt in open field populations.
The diamondback moth (Plutella xylostella), sometimes called the cabbage moth, is a moth species of the family Plutellidae and genus Plutella. The small, grayish-brown moth sometimes has a cream-colored band that forms a diamond along its back.
Other insects such as diamondback moth, Plutella xylostella, and cabbage looper, Trichoplusia ni, have evolved resistance to Bt sprays in the open field and green houses, respectively.