The correct answer is D. variable relationship.
A variable relationship is a relationship between two variables in which one variable changes in direct proportion to the other. In other words, if one variable increases, the other variable will also increase, and if one variable decreases, the other variable will also decrease.
In the context of the question, the relationship between change in activity and change in total costs is a variable relationship. This is because the total costs will change in direct proportion to the change in activity. For example, if the activity level increases by 10%, the total costs will also increase by 10%.
The other options are incorrect.
A fixed relationship is a relationship between two variables in which one variable does not change, regardless of the value of the other variable. In other words, the two variables are not related to each other.
A cause and effect relationship is a relationship between two variables in which one variable causes the other variable to change. In other words, the change in the first variable is the cause of the change in the second variable.
An ineffective relationship is a relationship between two variables in which the change in one variable does not have a significant impact on the change in the other variable. In other words, the two variables are not closely related to each other.