This problem is slightly tricky if you never seen such a technique before, but there are only three moves to it.
White would like to play at a to fill the group with a bulky five, but can't play there because it would be suicide. This is what we call oshi-tsubushi.
Bonus question: In this result diagram, how many points of territory does Black own?
NecRock?: I'd say three... (Or 3.5? I'm still not entirely clear on all that miai counting)
NecRock?: (if stones are added) A white play at a would threaten both marked black groups
xela: Right, so black needs to add a move to defend. That leaves three empty points on the board, plus the four white stones captured earlier, for a total of 7 points of territory. (This is the count part of miai counting, but doesn't go as far as the value of a move -- that's a whole other topic.)