What issue does a chemistry professor face if he surveys only students who attend his class to improve attendance?

The correct choice is undercoverage. This issue arises when certain individuals or groups within a population are not given a chance to be included in the sample. In this scenario, the chemistry professor is only surveying students who already attend his class, which means he is neglecting to include those students who do not attend. This group could possess different characteristics or reasons for their absence, leading to an incomplete understanding of attendance issues. By solely focusing on attendees, the professor doesn't capture the full picture of the student body, potentially missing valuable insights or different perspectives on attendance. This lack of representation is what defines undercoverage in sampling.