Q.The horizontal and vertical components of magnetic field of Earth are the same at some place on the surface of Earth. The magnetic dip angle at this place will be (A) $30^\circ$ (B) $45^\circ$ (C) $0^\circ$ (D) $90^\circ$

At most places on Earth, the resultant magnetic field $\vec B$ does not lie flat along the horizontal -- it dips at some angle below (or above) it. This angle, between the direction of the resultant field and the horizontal at a given place, is called the angle of dip or magnetic inclination, $\phi$. Resolving $\vec B$ into a horizontal component $B_H=B\cos\phi$ and a vertical component $B_V=B\sin\phi$ gives the simple relations $\tan\phi=B_V/B_H$ and $B=\sqrt{B_H^2+B_V^2}$; of the two components, $B_V$ can be measured comparatively easily.

The dip varies systematically with location: at the magnetic equator, the field is entirely horizontal ($B=B_H$, pointing from geographic south to north, $B_V=0$), so $\phi=0^\circ$; moving from the equator towards either magnetic pole, the dip increases steadily; at the magnetic poles themselves, the field is entirely vertical ($B=B_V$, directed straight up at the magnetic north pole and straight down at the magnetic south pole, $B_H=0$), so $\phi=90^\circ$ (or, by the sign convention used for the opposite pole, $270^\circ$). Lines joining places of equal dip on a magnetic map are called aclinic lines.