Which units represent bandwidth in fiber optics?

Bandwidth in fiber optics is primarily measured in frequency units. The most common units used for this purpose are megahertz (MHz) and gigahertz (GHz). These units measure how many cycles of a signal occur in a second, which directly relates to the data-carrying capacity of the fiber optic cable. Higher frequencies indicate a greater ability to transmit data simultaneously, thus determining the bandwidth of the fiber.

While the other options include units related to fiber optics, they do not pertain to bandwidth. Decibels (dB) and nanometers (nm) relate to power levels and wavelengths, respectively, rather than bandwidth. Micrometers (um) and millimeters (mm) refer to measurements of the physical dimensions of the fiber or its core and cladding rather than its data transmission capacity. Lastly, feet (ft) and meters (m) are standards of distance and not relevant to the measurement of bandwidth. Therefore, the correct choice emphasizes the relationship between bandwidth and the frequency of data transmission in fiber optics.