The second answer choice contains a vague pronoun reference. The pronoun them could refer to the walnuts or the shells.
After Emma separated the walnuts from the shells, she placed them in a wooden bowl.
The first answer choice shows a possible correction for the vague pronoun reference. The text has been rewritten so that the meaning is clear.
Emma placed the walnuts in a wooden bowl after she separated them from the shells.
Which of the following contains a vague pronoun reference?
A: Emma placed the walnuts in a wooden bowl after she separated them from the shells.
B: After Emma separated the walnuts from the shells, she placed them in a wooden bowl.