C

Step 1: Understand the mechanism of a nib split. A nib split allows ink to flow from the reservoir to the paper.
Step 2: Identify the properties of the split nib. The design creates two narrow channels that promote the movement of ink through the nib.
Step 3: The primary principle by which the ink moves in the nib split is known as capillary action, which is the ability of a liquid to flow in narrow spaces without external forces.
Step 4: Recognize that gravity flow (Option A) is not responsible for the ink movement in a horizontal plane; rather, it operates vertically.
Step 5: Diffusion of fluid (Option B) and osmosis of liquid (Option D) are not relevant here, as they involve different processes of movement in fluids.
Therefore, the correct answer is C: Capillary action.