Answer: 2) We removed the training wheels from my younger sister’s bike.
Explanation: a participle is a word formed from a verb and used as an adjective or a noun. In English participles are also used to make compound verb forms. From the given options, the sentence in which the word "training" functions as a participle, is the corresponding to option 2: We removed the training wheels from my younger sister’s bike, because it is used as an adjective that is modifying the noun "wheels."