In the English language, the normal order of words in a sentence is: subject + verb + object. The subject is the performer of the action (verb) and the object is the receiver of that action.
In the given sentence, the object "that someone had been following her" is placed before the verb "turned and saw", whereas it should be placed after. Hence the sentence requires change and option A is incorrect.
Option B is correct as the sentence follows the normal word order subject (She) + verb (turned and saw) + object (that someone...).
Option C is incorrect because the words are not in the required order, they're in the order: part of verb + subject + part of verb + object.
Option D is incorrect because the sentence is incoherent.