Calcium (Ca) has an atomic number of 20. What is calcium's ground-state electron configuration?
a) 1s²2s²2p⁶3s²3p⁶4s²
b) 1s²2s²2p⁶3s²3p⁶4s²3d²
c) 1s²2s²2p⁶3s²3p⁶4s²4p²
d) 1s²2s²2p⁶3s²3p⁶4s²4d²