Question

The teacher said they were going to grade the test over the weekend. Is it A. Pronoun shift in number B. Pronoun shift in person C. Ambiguous reference D. No error

Answer 1

Teacher = they?

No!

What do you think the error is?

Answer 2

Actually, I can see two possible answers -- depending upon the original meaning.

It could be a pronoun shift in number.

Or it could be an ambiguous reference if more than one person would be grading the test.