Which of the following contains a vague pronoun reference?

 - When Brian tried to explain the complicated equation to Steven, his head started spinning.
 - Steven's head started spinning when Brian tried to explain the complicated equation to him.
The first answer choice contains a vague pronoun reference. The pronoun him could refer to Brian or Steven.
When Brian tried to explain the complicated equation to Steven, his head started spinning.
The second answer choice shows a possible correction for the vague pronoun reference. The text has been rewritten so that the meaning is clear.
Steven's head started spinning when Brian tried to explain the complicated equation to him.