The correct answer is: C. Avoid the stress concentration.
A fillet is a rounded corner or transition between two surfaces. It is used to reduce stress concentration, which is a point where stress is concentrated in a material. Stress concentration can lead to fatigue failure, so fillets are often used in areas that are subject to high stress.
In the case of a rail section, the fillet is used to reduce stress concentration at the junction of the rail head and the rail base. This helps to prevent fatigue failure of the rail.
The other options are incorrect. Option A is incorrect because the fillet does not increase the lateral strength of the rail. Option B is incorrect because the fillet does not increase the vertical stiffness of the rail. Option D is incorrect because the fillet does not reduce the wear of the rail.