The correct answer is A. One.
A single-mode fiber optic cable is a type of optical fiber cable that has a core diameter of less than 10 micrometers. This small core diameter allows only one mode of light to propagate through the fiber, which results in lower attenuation and higher bandwidth than multi-mode fibers.
Multi-mode fibers have a core diameter of 50 micrometers or larger, which allows multiple modes of light to propagate through the fiber. This results in higher attenuation and lower bandwidth than single-mode fibers.
The number of modes that can propagate through an optical fiber is determined by the core diameter and the wavelength of light. The smaller the core diameter, the fewer modes that can propagate. The longer the wavelength of light, the more modes that can propagate.
In single-mode fibers, the core diameter is so small that only one mode of light can propagate. This is because the wavelength of light used in single-mode fibers is typically in the range of 1,300 to 1,550 nanometers. These wavelengths are much smaller than the core diameter of a single-mode fiber, which prevents multiple modes of light from propagating.
The single mode of light that propagates through a single-mode fiber is called the fundamental mode. The fundamental mode has a circular cross section and is the lowest-order mode that can propagate through the fiber.
The fundamental mode has the lowest attenuation and highest bandwidth of all the modes that can propagate through an optical fiber. This is why single-mode fibers are used in applications where high bandwidth and low attenuation are required, such as long-distance telecommunications.