Very good quiz! I'm surprised this hasn't been featured as it mixes well-known with little-known elements making it challenging but not impossible. I do like the order you chose for the answers. It's apparent to those who know the quirks of the periodic table, but it's perhaps not obvious to all, adding a bit of a challenge without making it undoable.
It may be more widely known because it has medical relevance; for example people on certain blood thinners need to avoid foods containing "Vitamin K" because potassium helps blood clot. Whereas sulphur's medical relevance is primarily fart-avoidance...
Vitamin K is a fat-soluble vitamin that has nothing to do with potassium. The "K" comes from the german for "coagulation". Sulfur is present in many antibiotics, and has about the same abundance in the human body as potassium.