What Was the "Solar Neutrino Puzzle

What was the "solar neutrino puzzle"?

A) Neutrinos fired at the Sun's core were not reflecting properly back to Earth.
B) The Sun's core was producing too many neutrinos.
C) Particle accelerators on Earth were interfering with the Sun's core.
D) The sun did not produce any neutrinos, indicating it cannot be powered by fusion.
E) Neutrinos from the Sun were spontaneously changing into unexpected types.