Answer:
For question a, the correct option is: This is an observational study because observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured.
For question b, the correct option is: This is an experiment because observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured.
For question c, the correct option is: This is an experiment because a treatment was deliberately imposed on the individuals in order to observe a possible change in the response or variable being measured.
For question d, the correct option is: This is an observational study because observations and measurements of individuals are conducted in a way that doesn't change the response or the variable being measured.
Step-by-step explanation: To give us a background on why the options above were chosen, let us see the difference between Observational study and Experimental study.
In an observational study, the researcher draws inferences from an independent variable that is not under his/her control. While in an experimental study, there in an intervention and the effects of the intervention are studied.
Therefore, in the questions a to d above, all we need to do is identity which variables were influenced and which were merely observed.
For question a, the variables were only observed.
For question b, the variables were influenced.
For question c, the variables were also influenced.
For question d, the variables were merely observed.
. We can only do that in this case, however, because the values are being multiplied by each other.