How was the solar neutrino problem explained by neutrino oscillations?

The solar neutrino problem was a long-standing discrepancy where the number of electron-neutrinos detected from the Sun was significantly lower than theoretical models predicted.12

Physicists proposed two primary explanations: either the models of nuclear energy generation in the Sun were incorrect, or neutrinos possessed a small mass, allowing them to change "flavor" as they traveled. The latter phenomenon, known as neutrino oscillation, implies that electron-neutrinos produced in the solar core transform into muon or tau neutrinos during their journey to Earth.123

This was confirmed in 2002 by the Sudbury Neutrino Observatory (SNO). By using heavy water as a detector, SNO was able to measure all three neutrino flavors. The results showed that while the count of electron-neutrinos remained lower than expected, the total number of all three neutrino types matched the original predictions. This discovery proved that neutrinos oscillate between flavors and, consequently, that they must possess a non-zero mass.23