Step 1: The naive filling order.
Chromium has atomic number 24. Following the usual Aufbau filling order and using argon as the core, we would first write its configuration as $[Ar]\,4s^2\,3d^4$.
Step 2: Why chromium does not actually settle here.
A $3d^5$ arrangement, with one electron sitting in each of the five d-orbitals, is unusually stable because of the extra exchange energy that comes from having several electrons with parallel spin. Nature prefers this stability enough that one electron slips out of the $4s$ orbital and moves into the $3d$ set.
Step 3: Writing the corrected configuration.
This shift changes $4s^2\,3d^4$ into $4s^1\,3d^5$, giving chromium the actual ground state configuration $[Ar]\,3d^5\,4s^1$, and this is the configuration exams and textbooks expect you to recall directly.
Step 4: Matching to the options.
The half-filled $3d^5$ along with a single $4s$ electron is the hallmark exception every chromium question is testing. \[ \boxed{[Ar]\,3d^5\,4s^1} \]