Final answer:
The correct preposition for the sentence is 'at', indicating a specific time. 'You must wake up at six o'clock tomorrow morning' is the complete and correct sentence. Prepositions are essential to show the relationships between different parts of a sentence.
Step-by-step explanation:
The correct preposition to fill in the blank in the sentence 'You must wake up ___ six o'clock tomorrow morning.' is 'at'. This preposition is used to indicate a specific time. So, the complete sentence should read 'You must wake up at six o'clock tomorrow morning.'
Prepositions are important in English language as they indicate relationships between different parts of a sentence. Using the wrong preposition can change the meaning of a sentence or make it sound incorrect. In this case, 'at' is the right choice because it specifies the exact time the student should wake up.
On the other hand, prepositions like 'by' would indicate a deadline before which something should happen, and 'in' could be used for nonspecific times during a period, such as 'in the morning.'