Guarding against invasion

All the framework links are secure so it is time for Black to apply the ninth directive. On which of the four points shown should Black play next?

A is a dubious invasion and, if it fails, ruins a sente endgame play on the first line, so this option can be quickly eliminated. B at best might take the corner in gote. C is worth a few points but doesn't help Black's framework. So D is the best option because it both prevents White from immediately invading the corner - which is bigger than C, and also strengthens the weakest framework link thus reducing the overall risk of any future invasion or encroachment.

The prevented invasion

The conservative potential sequence shown is the least Black has to worry about! It might be much worse. Perhaps White could somehow break into the side underneath the E-F link or alternatively grab a larger part of the corner, or even live in the corner whilst retaining sente. So Black should be very pleased to play at D. Given accurate play, this is a good preventative move. But perhaps more importantly, given the possibility of a unforeseen sequence preferential to White, this is a good move for eliminating the risk.