Are bananas radioactive?
Very slightly, yes. Bananas contain potassium, and a tiny fraction of natural potassium is radioactive. The dose from eating one is far too small to matter.
Going deeper
All natural potassium includes a small amount of potassium-40, a radioactive isotope. Bananas are rich in potassium, so they give off a tiny amount of radiation.
Your body keeps its potassium level steady, so eating bananas doesn't build up radiation inside you. You'd get a bigger dose from a short flight. Scientists sometimes jokingly use the "banana equivalent dose" to explain how small everyday radiation is.