Solve the Problem $\theta$ Is $f ( \theta ) = \sec \theta$

Multiple Choice

Solve the problem.
-For what numbers $\theta$ is $f ( \theta ) = \sec \theta$ not defined?

A) all real numbers
B) odd multiples of $\pi \left( 180 ^ { \circ } \right)$
C) integral multiples of $\pi \left( 180 ^ { \circ } \right)$
D) odd multiples of $\frac { \pi } { 2 } \left( 90 ^ { \circ } \right)$