a
$W=\Delta \mathrm{U}=\mathrm{U}_{\mathrm{f}}-\mathrm{U}_{\mathrm{i}}$
$\mathrm{W}=(-\vec{\mu} \cdot \overrightarrow{\mathrm{B}})_{\mathrm{f}}-(-\vec{\mu} \cdot \overrightarrow{\mathrm{B}})_i$
$=0+(\vec{\mu} \cdot \overrightarrow{\mathrm{B}})_i$
$=\left(100 \times 5 \times 10^{-3} \times 1 \times 10^{-3}\right) \times 0.2 \mathrm{~J}$
$=1 \times 10^{-4} \mathrm{~J}=100 \mu \mathrm{J}$