D.Total cost and the number of hats purchased
Step-by-step explanation:
When two variables are directly proportional, they increase or decrease at the same rate. This can be represented by the formula
Y = KX
where Y and X are the variables and K represents a constant.
If the pairs can be written in this form then they are directly proportional. In other words, if you can identify the constant K for each pair then they are directly proportional.
Lets take each pair.
A.Hours worked and money earned
This most likely would have a constant K which is equal to the amount of money paid per hour. This amount is usually a fixed amount.
B.Distance and time when speed is constant
Already the constant K is indicated as speed
C.Area and side length of a square
This could be a bit tricky as some people might see the relationship as
Area = The square of the side length
However, for this case the constant K is also taken to be equal to the side length.
Therefore, this can be written as
Area = K × Side length (where K is the length of the square)
D.Total cost and the number of hats purchased
This is a case of replicating option B without indicating a constant. If the option included "all hats are the same price" then we have a constant for the pairs, unfortunately it didn't and that makes it least likely to be directly proportional.