Knight's move cap
in reply is good for security; but in some contexts it will be a little too submissive.
Simply ignoring White's play invites , which is quite severe. One advantage the knight's move cap has is that this follow-up is stronger than in the case of the cap.
Therefore is an option: this warikomi play makes it quite hard for Black to decide between a and b next.
There is a good example of this tactic in game 8 of the Kamakura jubango.