Answer:
a) The number of people that are being studied.
Step-by-step explanation:
1) The (missing) options are:
a) The number of people that are being studied.
b) red blood cell count
c) height
d) none of the above
2) Explanatory variables are synonymical terms for independent variables.
In short when those variables are really not affected by other ones they are also called Explanatory Variables.
It is useful to make a distinction between response variable, and explanatory ones. The first one is affected somehow and it is the response Doris wants to find out.
3) Certainly, the sample or population of this research is absolutely free of prior influence over the final results. Since theoretically, the same research can be done over different samples and find other responses.
Then it's "a"
