Which of the following statements proves that Earth has a magnetic field?

$A$ dip needle lies initially in the magnetic meridian when it shows an angle of dip $\theta$ at a place. The dip circle is rotated through an angle $x$ in the horizontal plane and then it shows an angle of dip $\theta'$. Then $\frac{\tan \theta'}{\tan \theta}$ is

$A$ magnet hung at $45^{\circ}$ with the magnetic meridian makes an angle of $60^{\circ}$ with the horizontal. The actual value of the angle of dip is.

The relations among the three elements of Earth's magnetic field,namely horizontal component $H$,vertical component $V$,and angle of dip $\delta$ are,($B_{E} =$ total magnetic field):

Planets producing larger magnetic field have larger

Assume the dipole model for Earth's magnetic field $B$,which is given by:
$B_v = \text{vertical component of magnetic field} = \frac{\mu_0}{4\pi} \frac{2m \cos \theta}{r^3}$
$B_H = \text{horizontal component of magnetic field} = \frac{\mu_0}{4\pi} \frac{m \sin \theta}{r^3}$
where $\theta = 90^\circ - \text{latitude}$ as measured from the magnetic equator.
$(a)$ Find the loci of points for which the dip angle is zero.