In the following question, sentence given with blank is to be filled in with an appropriate word. Select the correct alternative out of the four and indicate it by selecting the appropriate option.
The insecticide is very powerful and can kill a wide ________ of insects.

The blank requires a word that indicates a broad range or coverage of different types of insects.
spectrum: Used to describe a wide range of positions, opinions, or a whole range of organisms that an agent (like a medicine or insecticide) can affect. "A wide spectrum of insects" is a standard and appropriate collocation.

volume: Refers to the amount or quantity of something, which does not fit contextually.

rainbow: Refers to a display of colors, used figuratively to show a colorful variety, but not appropriate for insecticides and insects.

plethora: Means a large or excessive amount of something; it does not standardly collocate with "wide" in this manner ("a wide plethora" is redundant/incorrect; we use "a plethora of").

Thus, spectrum is the most appropriate word.