4-4 point low approach low extension tenuki, diagonal attachment, 3-3 invasion
This is a fairly fundamental position in contemporary go. The white stones on the outside may well be weak, but both players ought to be well-informed about the status of the 3-3 invasion at . Can it live?
here must be counted as the typical answer. Alternatively, Black might block at a allowing White to connect out (nearly) at , which is purely defensive; Black might play at b which attempts to kill the corner but is bad shape; or play c, then a, which gives away eye shape to White's outside group.
Q: Can't black also answer at d?
Now is usual and is probably correct shape: to play at a allows White to push through and cut, and is potentially an overplay unless Black is very strong locally. But there are still difficult tactics possible after .
White has the idea of playing at here. White could simply live now, but this sets off tactics, assuming Black plays out of fighting spirit. White cuts at .
Black has the other option, of playing at first to see what White does. If then Black will set up the cut at b to break out. This is a tough fight.
If Black does block with here, White cannot quite live in the corner. On the other hand White now has a choice of sharp ways to play.
White and aren't quite life in the corner, yet. But the cutting point at a is serious for Black. Note that White at b has a big effect in life-and-death here.