A penetrant that spreads smoothly over a contaminated surface is exhibiting:

The correct response highlights the penetrant's ability to spread smoothly over a contaminated surface, which is characterized by its wetting ability. Wetting refers to the ability of a liquid to maintain contact with a solid surface, resulting in the formation of a thin film on that surface. This property is crucial in liquid penetrant testing because it affects the penetrant's ability to fill cracks and defects, thereby ensuring a reliable assessment of the material.

When a penetrant has good wetting ability, it can effectively displace contaminants on the surface, allowing it to penetrate into any surface irregularities. This is essential for revealing potential flaws within the material being tested. Without proper wetting, the penetrant may not adequately access these defects, leading to inaccurate results.

In this context, the other options do not directly address the penetrant's interaction with the surface in terms of spreading smoothly. Adhesion refers to the attraction between different substances, capillarity relates to the movement of liquids through narrow spaces due to surface tension, and viscosity is a measure of a fluid's resistance to flow. While these properties are important in certain contexts, they do not specifically describe the ability of the penetrant to spread over a contaminated surface.