Final answer:
Monochromatic light with wavelength 538 nm is incident perpendicularly.
Step-by-step explanation:
The correct answer is A. Perpendicularly.
When monochromatic light with a specific wavelength is incident perpendicularly, it means that the light is striking the surface at a 90° angle.
In this case, the light with a wavelength of 538 nm is incident perpendicularly, meaning that it is striking the surface directly and vertically.
For example, if you shine a flashlight directly on a wall, the light from the flashlight is incident perpendicularly on the wall.