Answer: The data in this scenario can be described as a sample.
A sample is a subset of a larger group, called a population. In this case, the student's purchase of a bag of Milk Chocolate M&M's represents a small portion of all the bags of Milk Chocolate M&M's that are available. The bags of M&M's that the student did not purchase are not included in the data.
On the other hand, a population refers to the entire group of individuals or items that we are interested in studying. In this scenario, the population would be all the bags of Milk Chocolate M&M's produced by the makers of M&M's.
Since the student's purchase represents only a specific bag of M&M's and not the entire population of bags, the data is best described as a sample.
Therefore, the correct answer is Sample.