Final answer:
The atom that would be expected to have a half-filled 4s subshell is d. chromium (Cr) due to the stability gained from having a half-filled 3d subshell.
Step-by-step explanation:
d. Chromium (Cr) is the atom that would be expected to have a half-filled 4s subshell. Unlike other elements, the electron configuration of chromium is [Ar] 4s¹3d5, rather than [Ar] 4s²3d¹, due to the stability gained from having a half-filled 3d subshell.
This stability is also observed in other elements like copper (Cu), where the electron configuration is [Ar] 4s¹3d¹⁰.