4219K → ___  +  0-1e A. 4218CaB. 4221CaC. 4220CaD. 4320Ca

The question is about nuclear reactions, specifically beta decay. In beta decay, a neutron in the nucleus of an atom decays into a proton, an electron, and an electron antineutrino. The electron and the antineutrino are ejected from the atom, and the proton remains in the nucleus, increasing the atomic number by 1.

The notation "4219K → ___ + 0-1e" represents the decay of a potassium-40 (40K) atom. The "4219" represents the mass number (number of protons + neutrons) and the atomic number (number of protons) of the atom, respectively. The "0-1e" represents the beta particle (electron) that is emitted during the decay.

Since the atomic number increases by 1 during beta decay, the product of the decay of a 40K atom would be an atom with an atomic number of 20 (19 + 1). The mass number remains the same because a neutron is simply converted into a proton.

Therefore, the correct answer is:

4219K → 4220Ca + 0-1e

So, the answer is C. 4220Ca.