Concept: An organic (polymer-based) coating protects steel mainly by acting as a barrier that keeps moisture, oxygen, and pollutants away from the metal. For a structure standing outdoors, the coating must survive two continuous attacks: it must stay firmly bonded to the steel, and it must resist the ultraviolet (UV) radiation in sunlight that breaks down polymers.

Why the correct answer is right: The prioritised combination is strong adhesion to the substrate and high resistance to UV radiation:

• Strong adhesion ensures the film does not blister, peel, or delaminate. If adhesion fails, moisture creeps beneath the coating and corrosion spreads underneath (undercutting), destroying protection even if the film itself is intact.
• High UV resistance is essential outdoors, because UV photons cleave polymer chains, causing chalking, embrittlement, cracking and loss of gloss and adhesion. UV-stable systems such as polyurethanes and fluoropolymers (or coatings with UV absorbers) retain their integrity for years of sun exposure.

Together these two properties give the long-term durability the question asks for.

Why the other options are wrong:

• Water solubility and rapid drying — a coating that can dissolve in water would simply wash off in rain, offering no lasting outdoor protection; this is the opposite of what is needed.
• High chemical reactivity and low cohesion to form a sacrificial layer — organic barrier coatings should be chemically stable and well-bonded; "low cohesion with steel" describes poor adhesion, and a truly sacrificial (galvanic) layer is a metallic zinc-type concept, not the mechanism of an organic barrier coating.
• Low viscosity and low electrical insulation — viscosity is only an application/handling property of the wet paint and does not determine the finished film's durability, while low electrical insulation is actually undesirable, since a good insulating film helps suppress the electrochemical corrosion cell.