Which of the following contains a vague pronoun reference?

 - Sarah asked Julia to make a flourless chocolate cake for their book club meeting because she has a gluten allergy.
 - Because Julia has a gluten allergy, Sarah asked her to make a flourless chocolate cake for their book club meeting.
The second answer choice contains a vague pronoun reference. The pronoun she could refer to Sarah or Julia.
Sarah asked Julia to make a flourless chocolate cake for their book club meeting because she has a gluten allergy.
The first answer choice shows a possible correction for the vague pronoun reference. The text has been rewritten so that the meaning is clear.
Because Julia has a gluten allergy, Sarah asked her to make a flourless chocolate cake for their book club meeting.