What is the source of a magnetic field?

If a positive ion is moving away from an observer with some acceleration,then the lines of force of magnetic induction will be

Two circular loops $L_1$ and $L_2$ of wire carrying equal and opposite currents are placed parallel to each other with a common axis. The radius of loop $L_1$ is $R_1$ and that of $L_2$ is $R_2$. The distance between the centres of the loops is $\sqrt{3} R_1$. The magnetic field at the centre of $L_2$ shall be zero if

Which of the following is a vector quantity?

The magnetic field at a perpendicular distance of $1 \,m$ from a long straight wire carrying a current of $1 \,A$ is:

$A$ circular coil of wire consisting of $100$ turns each of radius $9 \ cm$ carries a current of $0.4 \ A$. The magnitude of the magnetic field at the centre of the coil is $[\mu_0 = 12.56 \times 10^{-7} \text{ SI Units}]$.