Q.A place where the vertical component of Earth's magnetic field is zero has the angle of dip equal to (A) $0^\circ$ (B) $45^\circ$ (C) $60^\circ$ (D) $90^\circ$

[!TLDR] $B_V=0\Rightarrow\tan\phi=B_V/B_H=0\Rightarrow\phi=0^\circ$ -- the condition that holds everywhere o …

From $\tan\phi=\dfrac{B_V}{B_H}$ (Eq. 12.10), if the vertical component of the Earth's field is zero at some place, $B_V=0$, then $\tan\phi=0$, so $\phi=0^\circ$. This matches special case (3) of section 12.5: anywhere on the magnetic (great-circle) equator, the Earth's field is purely ho …

Set $B_V=0$ in $\tan\phi=B_V/B_H$ to get $\phi=0^\circ$, and identify this with the magneti …

Confusing 'vertical component zero' (dip $=0^\circ$, at the magnetic equator) with 'horizontal component zero' (dip $=90^\circ$, at a magnetic pole …