During beta decay what then theoretical particle was proposed to account for the discrepancy in mass/energy conservation? A neutrino B. electron C. positron D. proton E. neutron

A neutrino

Explanation:

Beta decay that releases the neutron, positron and the neutrino is called the beta plus decay. The release of the neutrino accounts for the discrepancy in the mass energy conservation. This is a type of radioactive decay called beta plus decay, where a proton is converted to a neutron accompanied by the release of a positron and a neutrino.

Beta plus decay requires energy. Isolated protons cannot cause this because the mass of the neutron is greater than the mass of a proton.