D

Step 1: Understand the definition of the magnetic meridian. The magnetic meridian is defined as the vertical plane that contains the Earth's magnetic field lines at a given location.
Step 2: In geographical terms, the magnetic meridian aligns with the North-South direction. Therefore, it represents a line that connects the magnetic north and south poles.
Step 3: Analyzing the options:
A: Point - Incorrect, as it refers to a specific location rather than a directional representation.
B: Horizontal plane - Incorrect, since the magnetic meridian is vertical.
C: Vertical plane - While this sounds similar, it does not specifically convey that it is aligned N-S.
D: Line along N-S - Correct, as it indicates the orientation of the magnetic meridian.
Therefore, the correct answer is D.