For direct buried optical fiber installations, which cable configuration is typically used to protect from moisture?

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Moisture protection is essential for direct-burial optical fiber, since water entering the cable path can travel along the fibers and cause attenuation or failure. A loose-tube cable design is ideal for outdoor burial because each fiber is housed in its own tube, and these tubes are filled with a water-blocking compound (gel or other swelling material). If water tries to migrate along the cable, the blocking material swells and forms a barrier, stopping moisture from spreading between tubes and along the length of the run. This combination of loose tubes plus water-blocking content is specifically engineered to resist moisture ingress in buried deployments.

The other options don’t address moisture protection in the same way. Copper twisted pair isn’t an optical fiber solution, so it doesn’t provide the necessary protection for fiber. Solid-core fiber without moisture protection would be susceptible to water ingress. A fiber ribbon in air ducts is an indoor/duct installation scenario and not suitable for direct burial.