Which of the following is correct about the given diagram?

Let's analyze all the options:

(a) All C are B (Incorrect)
Since All B are C and B is the Subset of C, Therefore this conclusion not possible.
(b) All A are C (Correct)
​Since All A are B and B is the Subset of C, Therefore this conclusion is  possible.
(c) All B are A (Incorrect)
​Since All A are B and A is the Subset of B, Therefore this conclusion not possible.
(d) All C are A (Incorrect)
​Since All A are B and B is the Subset of C, Therefore this conclusion also not possible.

Thus, the correct option is (b)