Q.Magnetic meridian is the plane (A) perpendicular to the magnetic axis of Earth (B) perpendicular to geographic axis of Earth (C) passing through the magnetic axis of Earth (D) passing through the geographic axis of Earth

[!TLDR] By definition, the magnetic meridian is the vertical plane containing the Earth's magnetic axis. [!ANSWER] (C) passing through the magnetic axis of Earth

Section 12.5 defines two reference planes at a place: the geographic meridian, the vertical plane perpendicular to the Earth's surface that passes through the geographic (rotation) axis, defining true north; and the magnetic meridian, the vertical plane, also perpendicular to the surface, that instead passes through the Earth's magnetic axis. The Earth's own resultant magnetic field at a place always lies along, or parallel to, this magnetic meridian plane -- which is exactly why a freely suspended compass needle settles into this plane. The angle between the two meridian planes at a place is the declination. Among the given options, only (C) correctly states that the magnetic meridian passes through the Earth's MAGNETIC axis (not the geographic axis, and not merely perpendicular to either axis). [!ANSWER] (C) passing through the magnetic axis of Earth

Recall the textbook definition of the magnetic meridian as the vertical plane through the Earth's magnetic axis, distinct from the geographic meridian (through the geographic axis).

Confusing the magnetic meridian (passing through the magnetic axis) with the geographic meridian (passing through the geographic/rotation axis), or thinking either meridian is defined as PERPENDICULAR to its axis rather than containing it.