I think it's

B. C23

The letter pattern is:

Take a letter, skip 5, take the next letter, skip 5, etc. When you're at the Z, just go back to Y, X, ... This means that we have I -> (H, G, F, E, D) -> C

The number pattern, as @Pat mentions in a comment on the other answer, is:

Prime numbers. So after 19 comes 23.

I think it's

B. $\boxed{\text{C 23}}$

The letter pattern is:

Take a letter, skip 5, take the next letter, skip 5, etc.  I.e., take 6 steps.  When you hit the Z, just go back to Y, X, ...  So the Y ⟹ U transition “bounces”: Y → Z → Y → X → W → V → U.  This means that we have I -> (H, G, F, E, D) -> C

The number pattern, as @Pat mentions in a comment on the other answer, is:

Prime numbers. So after 19 comes 23.