Why did scientists believe in the existence of the neutrino even though it wasn't discovered for several years after after the time of its prediction?

Question

a. They were able to observe their tracks in cloud chambers.

b. Scientists were confident in the law of Conservation of energy.

c. They were able to calculate exactly how much mass was being transported away from a parent atom during the gamma decay process.

d. The particles were in fact experimentally discovered first.

Answer 1

Choice b) is the best.    Neutrinos  leave no trace in cloud chambers.  Neutrinos have very little mass, so do not significantly contribute to the mass balance in gamma decay.  It is only fairly recently that direct detection of neutrinos has become possible.   Scientists are confident in conservation of both energy and momentum.   Neutrinos carry away both momentum and energy in weak decays.  Thus to make sense out of careful experimental studies of weak decays, a new, nearly mass zero particle had to be postulated - this was the neutrino.