Answer:
The verbs "can" and "could" are modal verbs that are used to express ability, possibility, permission, or request. "Can" is used in the present tense to express ability or permission in informal or casual situations. On the other hand, "could" is used in the past tense to express ability or permission in formal or polite situations.
In the given sentence, we can use "could" with the verb "say" to indicate that the baby was able to say 15 words when she was three months old. Therefore, the sentence would be: "When she was three months old, she could say 15 words."