The correct answer is: no article.
The word “light” can be used as a noun to refer to a source of illumination, or as an adjective to describe something that is bright or shining. In the sentence “Switch off the light,” the word “light” is being used as a noun, so it does not need an article.
If the sentence were “Switch off a light,” then the word “light” would be being used as an indefinite article, which means that it is referring to any light, not a specific light.
If the sentence were “Switch off the lights,” then the word “lights” would be being used as a plural noun, which means that it is referring to more than one light.
In conclusion, the correct answer is “no article” because the word “light” is being used as a noun in the sentence “Switch off the light.”