What should be used to provide a smooth, rounded insulating surface for conductors in raceways subjected to abrasion?

The correct choice is a fitting because fittings are specifically designed components that provide smooth, rounded surfaces for conductors within raceways. They help prevent abrasion of the conductors as they pass through bends and connections in the raceway system. The rounded design minimizes sharp edges and corners, which can cause wear and damage to the insulation of the conductors over time.

Fittings can also help in maintaining the integrity of the electrical system by ensuring that conductors are properly supported and protected from mechanical damage. Utilizing fittings in this manner aligns with the NEC's requirements for protecting conductors and ensuring their longevity and reliability within electrical installations.

The other options may serve specific purposes but do not provide the same level of protection and suitability for insulation against abrasion as fittings do. For example, while a protective sleeve can provide covering, it is not specifically designed to create smooth transitions in raceways. Similarly, insulative tape can help with insulation but lacks the structural support or shape necessary to effectively guard against abrasion when conductors are routed through raceways. Gel coating, while providing some degree of insulation or protection, does not offer the same mechanical protection as fittings do in a raceway application.