Answer: The answer is indeed:
After passing her driving test, Lisa felt a sense of lightness.
Please light the candles on the birthday cake.
Patrick did not take the test lightly and studied intensely.
Explanation:
Let's define the options:
Light is verb, meaning to make something burn or to make a place brighter by using some sort of lighting. So, it makes sense to say "light the candles".
Lightly is an adverb, so it should modify another adverb, a verb, or an adjective. In this case, it modifies the verb "take" in the structure "take the test lightly". "Take something lightly" means to not take something seriously.
Lightness is a noun, meaning - among other definitions - the lack of pressure. So, after taking the test, Lisa did not feel any pressure anymore, thus feeling a sense of lightness.
