The second answer choice contains a vague pronoun reference. The pronoun they is used without its antecedent.
The first answer choice shows a possible correction for the vague pronoun reference. They has been replaced with scientists.
How are scientists working to improve technologies that turn the sun's energy directly into electricity?
Which of the following contains a vague pronoun reference?
A: How are they working to improve technologies that turn the sun's energy directly into electricity?
B: How are scientists working to improve technologies that turn the sun's energy directly into electricity?