What is one nanosecond ?

Question

$\begin{array}{1 1} (a)\; \text{one hundred$^{th}$ of a second} \\ (b)\; \text{one million$^{th}$ of a second} \\ (c) \text{one billion$^{th}$ of a second} \\ (d)\; \text{one trillion$^{th}$ of a second} \end{array}$