Student or Learner
This is a test question, and a is the answer, but doesn't "could" also make sense as a lower possibility? Is it because it has to be a definite one?
Choose an awkward one => Answer :a
a)There are different types of media that could be used in the art of painting.(could=>can)
b)Artists do not have to limit themselves to a single medium, although they may favor one type over another.
c)Before choosing a specific medium, artists first decide on the theme of their painting and the subject they wish to paint.
d)They also consider the characteristics of various painting media, such as texture and drying time.