Which of the following statements is true regarding magnetic lines of force?

In an ionised sodium atom,an electron is moving in a circular path of radius $r$ with angular velocity $\omega$. The magnetic induction in $Wb/m^2$ produced at the nucleus will be

Two similar coils each of radius $R$ are lying concentrically with their planes at right angles to each other. The currents flowing in them are $I$ and $2I$. The resultant magnetic field of induction at the centre will be ($\mu_0 =$ Permeability of vacuum).

$A$ wire shaped in a regular hexagon of side $2 \text{ cm}$ carries a current of $4 \text{ A}$. The magnetic field at the centre of the hexagon is

$A$ wire has three different sections as shown in the figure. The magnitude of the magnetic field produced at the centre '$O$' of the semicircle by the three sections together is $(\mu_0 = \text{permeability of free space})$:

$A$ current carrying circular loop of radius '$R$' and a current carrying long straight wire are placed in the same plane. $I_c$ and $I_w$ are the currents through the circular loop and the long straight wire,respectively. The perpendicular distance between the centre of the circular loop and the wire is '$d$'. The magnetic field at the centre of the loop will be zero when the separation '$d$' is equal to: