What is the simplest form of the ratio 9:16?

The simplest form is when the ratio can no more be simplified further.

$\begin{array}{c}\frac{9}{16}=\left(\frac{3\times 3}{4\times 4}\right)\\ \end{array}$

Since there is no common factor in the numerator and the denominator,  $\frac{9}{16}$ is in the simplest form.