Electron capture decay is a decay process. In this process electron is captured and a proton is converted into a neutron. Hence, mass number remains same but atomic number is lowering by 1 in daughter atom.
K has atomic number 19. Hence daughter atom should have 19 - 1 = 18 as the atomic number. Hence, produced particle is Ar. but the mass number of Ar remains as same as 40.
The equation is,
₁₉⁴⁰K + ₋₁⁰e → ₁₈⁴⁰Ar
