Language changes all the time. Even though grammar changes m...

Language changes all the time. Even though grammar changes more slowly than vocabulary, it is not a set of unalterable rules. There are sometimes disagreements about what is correct English and what is incorrect. 'Incorrect' grammar is often used in informal speech. Does that make it acceptable? John Eastwood, author of Oxford Guide do English Grammar says: "Where there is a difference between common usage and opinions about correctness, I have pointed this out." This information is important for learners. In some situations, it may be safer for them to use the form which is traditionally seen as correct. The use of a correct form in an unsuitable context, however, can interfere with understanding just as much as a mistake. To help learners to use language which is appropriate for a given occasion, students must know that there are usages as formal, informal, literary and so on.
Only one alternative has no grammatical error. Which is it?