Answer:
D. the number of air conditioners sold per day in a city and the daily temperature
Explanation:
Let analyse all the possible answers:
A. the number of hats sold and the number of mittens sold in a city
They show no relationship towards each other, both of them are independent variables. If you buy hats, you can buy mittens or not it is ok.
B. the amount of butter and the amount of bread consumed by a family
They show no relationship towards each other, both of them are independent variables. If you buy bread, you can eat it with jams, without butter is fine.
C. the number of notebooks sold and the number of pens sold in a stationery store
They show no relationship towards each other, both of them are independent variables. I can use pencil to write on that notebooks, do not need to buy pens.
D. the number of air conditioners sold per day in a city and the daily temperature
They show a strong relationship towards each other, the daily temperature is the independent variable and it makes people want to buy air conditioners (dependent variable)
E. the number of hours a person can swim and the depth of the pool
They show no relationship towards each other, both of them are independent variables. The number of hours a person can swim depends on their ability.
So we choose D.