Which of the following is a weed killer?

Answer: (c) Herbicide

Herbicides (“herb” = plant) kill weeds, e.g. 2,4-D, glyphosate.

The idea

The ending “-cide” means killer. The first part of the word tells you the target.

Herbicide → weeds → Insecticide → insects → Fungicide → fungi → Rodenticide → rats and mice → Nematicide → nematode worms

Why the others are wrong

(a) Insecticides kill insects.

(b) Methyl salicylate is oil of wintergreen — a flavour and pain-relief rub.

Also know

Glyphosate is the most widely used herbicide in the world; it kills almost all green plants. 2,4-D is selective: it kills broad-leaved weeds and spares grasses such as wheat. A well-known weed of India is parthenium (congress grass).