Verified answer

B is a sentence fragment (it needs a subject, i.e. I am watching....).

A is in the imperative mood (the subject is understood as 'you').

C, the subject is 'it'.

D, again the subject is understood as 'you' ... ask your teacher for help.

B. You have to say what happens while you are watching the scary movie, in order to move this from a fragment to a complete sentence.

B. because it doesn't have a subject as I has a subject me c has its and d has you while b doesn't show who or what is watching a scary film.