The correct answer is B. On the test.
Step-by-step explanation
A prepositional phrase is a grammatical structure that includes a preposition such as on, in, at, and, a noun. An example of this structure is "On the test" which includes the preposition "on" and the noun "test". Additionally, prepositional phrases can be used in sentences to complete an idea. In the case of sentence 27, the best option is "on the test" because this has the structure of a prepositional phrase and it fits the context because the speaker is mentioning grades, which are common in tests.