Final answer:
The dependent variable in the experiment is the number of sit ups, as it is what is being measured and affected by the independent variable, which in this case is drinking lemonade.
Step-by-step explanation:
In the experiment to test the effect of drinking lemonade on the number of sit ups a student is able to perform, the dependent variable is the number of sit ups (option d). This is because the dependent variable in an experiment is the variable being tested and measured, and in this case, it is the performance of sit ups which potentially changes in response to the independent variable, which is the act of drinking lemonade. In the doughnut study example, the dependent variable is the weight gained (option d). Similarly, in Lind's experiment, the dependent variable is the improvement/no improvement in scurvy symptoms, since it relies on the independent variable, the daily supplement received by the participants. These examples illustrate the relationship between an independent variable that is manipulated and a dependent variable that is observed to see how it is affected by the change in the independent variable.