ANSWER
[tex] \frac{\pi}{2} \: radians[/tex]
EXPLANATION
To convert from degrees to radians, we multiply by:
[tex] \frac{\pi}{180 \degree} [/tex]
We want to convert 90° to radians.
This implies that,
[tex]90 \degree = 90 \degree \times \frac{\pi}{180 \degree} = \frac{\pi}{2} \: radians[/tex]
The last option is correct.
