Potassium has a radioactive isotope, $^{40}K$ - VCE - SSCE Chemistry - Question 18 - 2005 - Paper 1

During the beta decay process, a neutron in the $^{40}K$ nucleus is transformed into a proton, which results in the emission of a beta particle (electron). As a result, the atomic mass remains the same (40), but the atomic number increases by 1, leading to the formation of a new element.

The reaction can be summarized as follows:

u}$$ Thus, the nucleus produced is $^{40}Ca$, which corresponds to option D.