The emission of beta particles in radioactive decay is evidence for the existence of A quarks B electrons C gluons D neutrinos E bosons. - Scottish Highers Physics - Question 9 - 2015

The emission of beta particles during radioactive decay is primarily evidence for the existence of neutrinos. In beta decay, a neutron in an unstable nucleus transforms into a proton while emitting a beta particle, which is an electron in the case of beta-minus decay or a positron in beta-plus decay. Along with this beta particle, a neutrino is also emitted to conserve momentum and energy during the decay process.

Neutrinos are nearly massless and interact very weakly with matter, which is why they are difficult to detect. Their existence was postulated to account for the missing energy and momentum in beta decay processes. Hence, the presence of beta particles indicates the involvement of neutrinos in the decay process.