Final answer:
The correct answer is inaudible.
Step-by-step explanation:
The correct answer is inaudible.
Inaudible means that something cannot be heard or is not loud enough to be heard. In this case, the student's grandfather spoke so quietly that his words were almost inaudible, meaning they were difficult to hear.
Here's an example sentence using inaudible: When the teacher whispered, her words were inaudible to the students in the back of the classroom.
Learn more about The meaning of the word inaudible