Answer: have been
Step-by-step explanation:
Well, when you first do process of elimination, figure out which one(s) sound right. When doing this, we can eliminate c and d. However, to some people (myself included) a and b both sound right. So when you look back to the subject, you realize there are multiple ones, aka plural. I remember that have been is for plural and has been is for singular.
This may not be the academic explanation, but this is how I would answer a problem like this. It has always helped!