This position is from a few moves later in the game that appears in Beginner Move Function Problem 4. Match the numbered idea with the lettered move.
1. Flatten out/prevent expansion of Black moyo. c
2. Link stones before they become isolated. a
3. Attack Black stone within White sphere of influence. b
4. Get a foothold near Black thickness. e
5. Invade Black sphere of influence. d
b looks best to me. Obsoletes a, certainly. d looks suicidal. e looks dull. Against c, black can extend from the bottom left stone, and black threatens to take more territory on the bottom than white threatened on the left.
5 kyu analysis:d is ok because white can run out, or cross under at f. b is good because it prevents a move by black at g, which would make the white stone on the lower side lonely. We could say that white a is bad because it induces black g.