The correct answer is C. By reducing the bend.
Multimode fibers are made up of multiple paths for light to travel through, which can cause signal loss if the fiber is bent too sharply. Reducing the bend radius of the fiber will reduce the amount of loss.
Option A is incorrect because designing fibers with large relative refractive index differences will not reduce macro bending losses. In fact, it may actually increase them.
Option B is incorrect because maintaining the direction of propagation will not reduce macro bending losses. The light will still be able to travel through the fiber, but it will be more likely to be lost if the fiber is bent too sharply.
Option D is incorrect because operating at larger wavelengths will not reduce macro bending losses. The wavelength of light does not affect the amount of loss due to macro bending.
In conclusion, the correct answer is C. By reducing the bend.