If $\cos \theta =\frac{a}{b}$, then $\mathrm{cosec }\; \theta=$

Question

$\begin{array}{1 1} (a)\; \large\frac{b}{\sqrt{a^2-b^2} } \\ (b) \; \large\frac{b}{\sqrt{b^2-a^2}} \\ (c)\; \large\frac{a}{\sqrt{a^2-b^2} } \\ (d)\; \large\frac{a}{\sqrt{b^2-a^2}} \end{array}$