c
$(c)$ Given magnetic field due to a square loop $D A B C A$ at centre of cube is $B$.
Now, loop $D A E F G C D$ can be viewed as super position of three square loops as shown below.
So, net field at centre $=$ resultant of fields of these three loops
$\therefore \quad B _{\text {net }}=B \hat{ i }+B \hat{ j }-B \hat{ k }$
Magnitude of resultant field is
$\left| B _{\text {net }}\right|=\sqrt{B^{2}+B^{2}+B^{2}}=\sqrt{3} \cdot B$
