What is an herbicide?

Question

$\begin{array}{1 1} (a)\;\text{a substance or mixture of substances used to kill a pest} \\ (b)\; \text{which is used to kill weeds, especially an pavements and railways} \\ (c)\; \text{which is used to rid farms of pests which damage crops} \\ (d)\; \text{used to kill insects and pests} \end{array}$