A

Step 1: Understand the concept of capillary action, which is the phenomenon of liquid rising in a narrow space, such as a wick.
Step 2: Recognize that this effect is primarily due to surface tension, which is the cohesive force between the liquid molecules, allowing it to 'climb' the wick against gravity.
Step 3: Option B, while partially true, does not explain the fundamental physical mechanism, as it is the surface tension which plays a dominant role.
Step 4: Option C refers to diffusion, which is not the mechanism at play.
Therefore, the correct answer is A due to the surface tension of the oil.