Final answer:
Dr. Kramer has relied on a census.
Step-by-step explanation:
The correct answer to the question is c. relied on a census.
When all students in Dr. Kramer's two classes complete the survey, he has conducted a census. A census is a type of survey in which data is collected from the entire population rather than a sample. This can provide a more accurate representation of the population since every member is included.
By using a census, Dr. Kramer ensures that all students are represented in his study, increasing the external validity of his findings. However, it is important to note that relying solely on a census may limit generalizability to populations outside of Dr. Kramer's two classes.