Answer:
My grandfather spoke so quietly that his words were almost inaudible.
Explanation:
We are supposed to use the infixes (formative elements of a word) to form a word that will complete the sentence properly. According to the sentence, the speaker's grandfather spoke quietly. We can safely assume it was difficult for the speaker to hear the words his grandfather said. In that case, we need "aud", which is connected to the verb "hear". Since it was not possible to hear, we also need "in-", meaning "not", and "-ible", meaning "can be done". Together, they form the adjective "inaudible", which means "unable to be heard".
When we complete the sentence, it becomes:
My grandfather spoke so quietly that his words were almost inaudible.
