Which of the following d-block elements has half-filled penultimate as well as valence subshell?

The correct option is D Cr
Electronic configuration for :
$Cu:\left[Ar\right]3d^{10}4s^1$
$Au:\left[Xe\right]4f^{14}5d^{10}6s^1$
$Ag:\left[Kr\right]4d^{10}5s^1$

$Cr:\left[Ar\right]4s^{1}3d^5$
Here in chromium, valence shell $4s^1$.Penultimate shell $3d^5$


Chromium has half filled penultimate as well as valence subshell. Hence option (d) is correct.