Think of blotting paper under a microscope. It looks like a mesh of thin fibers with narrow air gaps running between them. Each gap works like a tiny capillary tube.
Capillary rise happens when a liquid wets the walls of a narrow tube. The liquid climbs up because the adhesive pull between liquid and tube wall beats the liquid's own cohesive pull.
Ink wets the cellulose fibers of the paper easily, so it gets drawn into the narrow gaps and spreads sideways and inward through the sheet.
This is exactly how capillary action works, and it explains why the ink is soaked up quickly and evenly.
A siphon needs a closed tube filled with liquid and a height drop on one side. Blotting paper has neither, so siphon action does not apply here.
Diffusion works on a concentration gradient inside a single phase and is far too slow to describe ink vanishing into paper in seconds.
Viscosity only tells us how easily the ink itself flows. It plays a small supporting role but is not the reason the ink gets pulled into the paper.
So the process at work is capillary action, matching option C.