Question

identify the parameter(s):
the mean amount of medication taken.
the number individuals who take the medication.
the proportion of those who take the drug who get a headache.

Explanation:

A parameter describes a population characteristic. The proportion of all individuals who take the drug and get a headache is a population parameter (while a statistic would be from a sample). The mean amount of medication taken could be a parameter if referring to the population's mean, but the option about the number of individuals is a count (not a parameter in the statistical sense of describing a characteristic like proportion/mean). The proportion of those who take the drug and get a headache is a parameter as it's a population - related characteristic (e.g., if we consider all people who take the drug, this proportion is a parameter describing that population).

Answer:

The proportion of those who take the drug who get a headache.