The correct answer is C. cross-section.
A rail is a long, thin piece of metal that is used to support the weight of a train. The cross-section of a rail is the shape of its cross-section, which is the area that is perpendicular to the length of the rail. The cross-section of a rail is important because it determines the strength of the rail. A rail with a larger cross-section will be stronger than a rail with a smaller cross-section.
The other options are incorrect because they are not relevant to the strength of the rail. The length of a rail does not affect its strength, and the weight of a rail is only important if the rail is not strong enough to support the weight of the train. The weight per unit length of a rail is also not relevant to the strength of the rail.
In conclusion, the correct answer is C. cross-section.