For the electron configuration of the transition element chromium (Cr), which has an atomic number of 24. This means a neutral chromium atom has 24 electrons.

For 24 electrons, the standard filling order would be:
$1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^4$
Let's count the electrons: $2+2+6+2+6+2+4 = 24$.

Chromium is an exception to the Aufbau principle. Atoms tend to achieve greater stability with half-filled or fully-filled d-orbitals. In the case of chromium, promoting one electron from the $4s$ orbital to the $3d$ orbital results in a half-filled $3d$ subshell ($3d^5$) and a half-filled $4s$ subshell ($4s^1$), which is more stable than $4s^2 3d^4$.

So, the actual electron configuration for chromium is:
$1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5$
Let's count the electrons again: $2+2+6+2+6+1+5 = 24$.

• (a) $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^4$ (This is the expected configuration, but not the actual stable one for Cr).
• (b) $1s^2 2s^2 2p^6 3s^2 3p^6 3d^6$ (Incorrect, $4s$ orbital is missing and electron count is 24, but order is wrong).
• (c) $1s^2 2s^2 2p^6 3s^2 3p^6 3d^4 4s^1$ (Incorrect electron count: $2+2+6+2+6+4+1 = 23$).
• (d) $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5$ (This matches the actual stable configuration for chromium).

The correct option is d).
$\boxed{\text{d) } 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5}$