Final answer:
The phrase 'under the car' in the sentence acts as an adverb because it describes where the soccer ball was kicked, which is providing additional information about the verb 'kicked'.
Step-by-step explanation:
The question asks to identify whether 'under the car' acts as an adjective or an adverb in the sentence 'The boy kicked the soccer ball under the car.' In this sentence, 'under the car' is a prepositional phrase that is providing more information about where the soccer ball was kicked. Hence, it is functioning as an adverb because it modifies the verb 'kicked' by specifying the location where the action took place.
Using examples from English grammar, when a phrase tells us where, when, how, or to what extent an action happened, it is generally acting as an adverb. However, if a phrase describes a noun, it would be considered as acting adjectivally.