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.
I do not like b and would prefer a. It makes a 100% save group facing blacks upper thickness. If black answers a with g, its group still is not save and w lower hoshi stone is not in real danger. White b induces black to split the white 3-3 from whites other stone on the right, forcing the latter to seek life in front of blacks thickness.