The first answer choice contains a vague pronoun reference. The pronoun they is used without its antecedent.
The second answer choice shows a possible correction for the vague pronoun reference. They has been replaced with the servers.
At lunchtime, Mike can often be found reading at his favorite café, where the servers provide unlimited coffee refills.
Which of the following contains a vague pronoun reference?
A: At lunchtime, Mike can often be found reading at his favorite café, where they provide unlimited coffee refills.
B: At lunchtime, Mike can often be found reading at his favorite café, where the servers provide unlimited coffee refills.