The correct answer is A. All of the kids who buy toys.
A population is the entire group of individuals or objects that a researcher is interested in studying. In this case, the researcher is interested in studying all of the kids who buy toys. The other choices are all samples, which are subsets of the population.
B. 227 rich kids is a sample because it only includes kids who are rich.
C. 228 boys age 7 - 15 is a sample because it only includes boys who are between the ages of 7 and 15.
D. 235 kids from age 10 to 15 is a sample because it only includes kids who are between the ages of 10 and 15.
Therefore, the only choice that represents a population is A. All of the kids who buy toys.