Answer: Option B
:
B
The question is why Kant introduced the term noumena. The question doesn’t ask you how Kant defends the existence of noumena.
Look at these lines -
"Kant argued that they must exist because phenomena are appearances, thus they must be appearances of something and that something must be noumena."
This tells why option (a) is wrong.
These lines will tell you why Option (b) is only correct
"Kant was aware that the external world was limited by our understanding, and noumena are introduced partly as proof of this (i.e. noumena are the limits of human understanding).
It is the noumena that play the role of independent existences beyond the thinking subject."