Molly, who attends a large, diverse university, spends most of her time with friends who share her ethnicity, attitudes, and opinions. this probably means that molly and her friends make what kind of sample of all the students at the school?

The answer is "an unrepresentative sample".

Biased Sample, otherwise called the Unrepresentative Sample is viewed as a weak analogy.

This fallacy (a misstep, and a legitimate error is a slip-up in thinking) is framed when a man reaches a determination of a populace in light of a measure that is one-sided or biased.  

The sample is biased or Unrepresentative somehow because of not having been picked arbitrarily from the population.