Answer:
d. rate
Step-by-step explanation:
Rate of speed is the velocity with which you pronounce words per minute, this means that the rate os speed is a counting of how many words can you say per minute, in this case a good speed rate to be perfectly understandable should be between 120 and 140 words per minute, more than that can be hard to understand. And lower than that can be tiring and not appealing.