which of the following is defined a) tan 90 b ) cot 0 c ) cosec 90 d ) sec 90

tan 90 = $\frac{\sin 90}{\cos 90} = \frac{1}{0}$ is not defined

cot 0 = $\frac{\cos 0}{\sin 0} = \frac{1}{0}$ is not defined

cosec 90 = $\frac{1}{\sin 90} = \frac{1}{1} = 1$

sec 90 = $\frac{1}{\cos 90} = \frac{1}{0}$ is not defined

Thus, only cosec 90 is defined.