A well-watered plant standing in good light has grown to a normal size for its age, but its leaves have turned yellow. A shortage of which mineral best explains that, and why?

Erklärung

Leaves look green because of the pigment in them, so when a plant cannot build enough of that pigment the green fades and yellow shows through. The question rules out the everyday explanations by supplying plenty of water and light, and it rules out the protein-building mineral as well, because a plant short of that one falls behind in size rather than reaching a normal one.