The sentence that has en error and has to be edited is option C) "At that point, I stated that either Martha or Emily had to begin their presentation on the video."
In this sentence, the pairing "either...or..." is used. When using this expression, and if both elements are in singular, then the singular form should be used. The correct sentence is "At that point, I stated that either Martha or Emily had to begin her presentation on the video."
