Final answer:
The sentence should be completed with 'in,' making it 'Beth starts school in August.' Prepositions such as 'in' are used to indicate time periods and link nouns or pronouns to other words, providing context such as time or location.
Step-by-step explanation:
The correct preposition to complete the sentence "Beth starts school in August" is 'in.' Prepositions are used to show the relationship of a noun or pronoun to another word in the sentence. In this context, 'in' is used because it indicates a period of time when something happens.
Prepositional phrases, like 'in August,' function as adverbials, providing more information about time, location, or manner. For instance, the sentence "By starting my general studies classes last spring, I had the opportunity to take classes in my major this fall." uses 'by' as a preposition to indicate how the opportunity was presented.
It is important to choose the right preposition to convey the correct meaning. Other examples of commonly used prepositions include 'about,' 'through,' and 'aboard,' which all serve to give additional context to the subjects they are associated with.