$\begin {array} {1 1} (a)\;\large\frac{\mu_o}{4 \pi} Q\large\frac{\overrightarrow V \times \hat r}{r^3} & \quad (b)\;\large\frac{\mu}{4 \pi} Q\large\frac{\overrightarrow V \times \hat r}{r^2} \\ (c)\;\large\frac{\mu_o}{4 \pi} Q\large\frac{\overrightarrow V \times \{\hat r \times ( \overrightarrow V \times \hat r) \}}{r^2} & \quad (d)\;\large\frac{\mu_o}{4 \pi} Q\large\frac{\overrightarrow V \times \overrightarrow r}{r^3} \end {array}$