Whenever a liquid wets a solid surface well, meaning it tends to spread rather than bead up, squeezing that liquid into a narrow gap between two closely spaced solid surfaces creates a suction like pressure that pulls the liquid further in on its own. That is exactly the situation in brazing, since molten filler is chosen to wet the base metal, and the joint clearance is deliberately kept tiny, usually a few hundredths of a millimetre, so this pulling effect is strong. Neither gravity nor a viscosity gradient is doing this work, since the filler often has to flow sideways or even upward into the gap. The mechanism responsible is capillary action, option (C).