b
Magnetic force is given by
$\mathrm{dFm}=\mathrm{i} \int \mathrm{d} \vec{\ell} \times \overrightarrow{\mathrm{B}}=\mathrm{i} \int \mathrm{d} \ell(-\hat{\mathrm{j}}) \times(-4 \hat{\mathrm{k}})=4 \mathrm{i} \int \mathrm{d} \ell \hat{\mathrm{i}}$
since $\ell$ and $B$ are perpendicular so
$\mathrm{df}_{\mathrm{m}}=8 \int \mathrm{d} \ell \hat{\mathrm{i}}=8 \times 4 \hat{\mathrm{i}}=32 \hat{\mathrm{i}}$
