The correct option is D a ratio between two different units with a denominator of 1
A unit rate means a rate for one of something. We write this as a ratio with a denominator of one.
For example, if you ran 80 meters in 10 seconds, on average you ran 8 meters in 1 second. Here, the ratio 8 meters in 1 second is a unit rate.
Also units of both the quantities are different i.e. meter and second.