SAT Math: Recognizing the Most Unreliable Factor in Survey Results?

Associates of a new coffee shop want to determine the city's most popular coffee shop. Many respondents said they were too busy to answer the survey. For that reason, the associates were only able to obtain 115 results. Which of the following factors most makes their data unreliable?

Product TypeSAT Math
SATSAT Math
SAT MathProblem Solving and Data Analysis
Statistics and Probability
Test PrepSAT Math