Concept:
Physics - Magnetism and Earth's Magnetic Field.
Step 1: Understand the Magnetic Meridian.
The magnetic meridian is the vertical plane passing through the magnetic axis of a freely suspended magnet. The Earth's total magnetic field ($B$) lies entirely within this plane.
Step 2: Analyze the components of the field.
In a plane perpendicular to the magnetic meridian, there is no component of the Earth's horizontal magnetic field ($B_{H}$).
Step 3: Recall the Time Period formula.
The time period of vibration for a magnet is:
$$ T = 2\pi\sqrt{\frac{I}{MB_{H}}} $$
Step 4: Apply the condition.
In a plane perpendicular to the meridian, the effective horizontal component becomes zero ($B_{H} = 0$).
$$ T = 2\pi\sqrt{\frac{I}{M \times 0}} = \infty $$
The needle will not vibrate; it will remain stationary in any position it is placed.