Final answer:
The ambiguous words in the sentence are "she" and "her" because it's unclear if they refer to Riley or her sister; thus, the sentence could be clarified by specifying the subject.
Step-by-step explanation:
In the sentence "Riley and her sister liked playing outside because she enjoyed riding her bike," the two ambiguous words are "she" and "her." The ambiguity arises because it's not clear whether "she" and "her" refer to Riley or to her sister. Ambiguity occurs when a word, phrase, or statement contains more than one meaning, leading to uncertainty in communication. In trying to avoid ambiguity, one could rewrite the sentence to specify to whom the pronouns refer, ensuring that the meaning is clear.