Which of the following contains a vague pronoun reference?

 - Lucy asked Rachel to volunteer at the event, so Rachel's absence today is a surprise.
 - Lucy asked Rachel to volunteer at the event, so her absence today is a surprise.
The second answer choice contains a vague pronoun reference. The pronoun her could refer to Lucy's or Rachel's.
The first answer choice shows a possible correction for the vague pronoun reference. Her has been replaced with Rachel's.
Lucy asked Rachel to volunteer at the event, so Rachel's absence today is a surprise.