Which form of decontamination is effective only on flat surfaces?

The correct answer is absorption. This process is specific to flat surfaces because it involves the uptake of contaminants into a material rather than on the surface. In the context of decontamination, absorption refers to a process where contaminants are absorbed into a porous material. This technique is particularly useful for spill cleanup on surfaces such as floors, where the absorbent material can soak up liquids, hence effectively reducing the contamination on that flat surface area.

While other forms of decontamination exist, they are not limited to flat surfaces in the same way absorption is. For instance, adsorption involves the adherence of contaminants to the surface of a material rather than penetration into it, which can happen on various surface types, not exclusively flat. Mechanical decontamination relies on physical removal techniques (like scrubbing or washing), suitable for multiple surfaces, and deactivation generally involves chemical processes aimed at neutralizing hazardous materials rather than focusing on specific surface types. This specificity in absorption makes it uniquely effective for flat surfaces, highlighting why it is the correct answer.