Answer:
A. Unit rate.
Explanation:
We have been given a statement and we are supposed to choose the correct option for our given statement.
Statement:
A constant of proportionality can also be considered a(n) .
Since we know that constant of proportionality represents the amount by which two proportional quantities vary.
Two quantities are directly proportional, when they are in form
, where k represents the constant of proportionality.
We can represent this proportion also as:

Since k represents the quotient of y and x, so the value of k represents the unit rate for two quantities y and x, therefore, a constant of proportionality can also be considered an unit rate.