1. Attempt to run out to the center with a weak stone.
2. Destroy shape, threaten to capture stones.
3. Enclose potential territory, create an open skirt for the opponent.
4. Kikashi - a forcing move.
5. Endgame move to reduce territory.
6. Create a loose, large-scale moyo.
SnotNose: I'll jump in with my thoughts. (Don't read if you want to solve the problem on your own!!!).
In this position, white has built up substantial power (thickness, influence) and black has the lead in territory. Black needs to play so as reduce White's power and increase his own. That is, Black needs to aim to reballance the board situation. I discuss each move from this point of view.
g seems too small right now. If white simply protects, to form a wall extending to the left edge, Black ends in sente and can then play at b (or maybe one line higher) forcing white to defend in overconcentrated shape. So, White should ignore g. Doing so will enable black to increase his territorial lead but not by much. White, meanwhile, will take a few large points elsewhere, solidifying his moyo and power, and dominate the game.
d also seems small. Moreover, I think white can protect in sente as follows
e seems like it is asking for trouble. White should be able to build up enough power through attacking to ensure that black gets no substantial territory elsewhere.
c only makes sense if Black will then play e, which I've already argued is not good. If Black is not going to escape with e, then playing c only gives white another stone and limits Black's endgame potential on the lower edge.
b is on the third line, going for more territory and doesn't do anything to address the main theme here--white's moyo. I think it is the wrong timing/wrong direction.
f looks playable. A Black move at f is a probe of sorts. If white defends the bottom, Black can continue to force white to defend by making kikashi against the bottom from above (i.e., more moves from the f direction). This will build up power for Black, which will help him build territory at the top and/or attack any white invasion there. Black can also take his sente endgame moves at the bottom edge early, forcing White to defend the bottom more. Finally, when he must take gote, Black should do so by taking a large point at the top (like a).
On the other hand, if White tries to attack a black play at f by first playing at the top, there is enough room in the center for Black to live and destroy White's moyo. Note, should White choose this course of play, only then is a play by Black at c and e (or thereabouts) okay. Because then Black will need to connect to his bottom stone to ruin White's territory. This is the implicit threat behind f.
a is undeniably big and has the virtue of at least facing the white moyo (though from very far away!!). But once Black has played a it gives white the initiative to expand his moyo.
Alex Weldon: Full marks. Something like f is what I'd play too, although the exact point at which it's best to reduce is a very advanced question indeed, that I'm not qualified to answer.
In the actual game, I was White. Black played the kikashi at c (which I answered of course) then the wide corner enclosure at b, undoubtedly a bad move. I answered at tengen (although now I'm strong enough to realize that something closer to f is better for strengthening the moyo). Black then tried to deeply invade the moyo a move too late, I killed him, and he lost miserably.