Answer:
D. It shows that Sari thought the test was easy.
Step-by-step explanation:
Idioms are expressions with a meaning that isn't deducible from those of the individual words they contain. That is why these phrases should not be taken seriously.
In the given passage, we have an idiom a piece of cake. When someone says that something was a piece of cake, they actually mean that something was easy, simple to accomplish. As the test turned out to be easy, Sari was positive that she did well.