Question

what is the variable being examined for individuals in the population(s)?
whether or not an individual takes the correct dosage of the drug.
whether or not an individual who takes the drug gets a headache.
whether or not an individual who does not take the drug gets a headache.
is it categorical or

Explanation:

To determine the variable, we analyze the context (likely a drug - related study). The variable should focus on the outcome for those taking the drug. The option "Whether or not an individual who takes the drug gets a headache" is about the response (headache or not) of individuals who take the drug, which is the relevant variable for a study on the drug's effect on headaches. The other options either focus on dosage (not the outcome variable here) or on those not taking the drug (while the study likely focuses on those taking it to see the effect).

Answer:

Whether or not an individual who takes the drug gets a headache.