Answer:
The answer is C.
Step-by-step explanation:
Population and sample are terms used in researchs to describe the different sets of subjects and helps distinguish them.
In the example given in the question, the research is the email survey for the people who bought tablets. Although the total number of tablets sold is 570, the survey is only sent to 300 of them which were randomly selected so the total population of the survey is 300. And out of that 300, only 138 responded to the survey email and are the only ones that are qualified for observation and acquiring research data. Therefore the sample of the survey is 138.
I hope this answer helps.