Fill in all the gaps with any word that makes sense, and then press "Check" to check your answers. If what you wrote creates a meaningful phrase with the words that follow, that word will be a preposition. Only single words will count in this exercise although certainly multi-word prepositions like "out of" and "away from" exist. If your response is not accepted, ask the instructor, for it is not possible to foresee all possible correct answers.
Prepositions are relational words that are placed before nouns and pronouns (pre-positioned) to make phrases. These phrases express location, time, and a few other aspects of the topics we discuss:
BEHIND an elephant
UNDERNEATH the cup
DURING that concert
OF our home
BY Paul and Virginia
Relationships between things are important; it makes a critical difference if Harry is seated BEFORE the fire or NEAR it rather than IN it or OVER it. However, these relationships, which prepositions describe, are by no means the most important ones. It makes a still greater difference whether it is Harry's dinner or Harry himself that is located over the fire! This most important information is expressed not in prepositional phrases but in the building blocks that constitute a sentence: the subject, verb, and complement (usually an object).
Discovering these building blocks is how you can begin to tune up the sentences to make them more effective. You are asked to recognize prepositions because even though the phrases they create often account for most of the words in a sentence, these at best amount to the nuts and bolts, never the real building blocks. By the process of elimination, you can find the structural elements more easily by first removing the prepositional phrases from consideration. If you then discover that the remaining words do not express your most important ideas, you should revise the sentence.