Final answer:
ADSL is the DSL variant with a distance limitation of 18,000 feet between a DSL modem and its DSLAM.
Step-by-step explanation:
The DSL variant that has a distance limitation of 18,000 feet between a DSL modem and its DSLAM is ADSL (Asymmetric Digital Subscriber Line).
ADSL is a type of DSL that allows users to receive data at a higher speed than they can send it. The asymmetry is designed to prioritize download speeds for activities like web browsing, streaming, and downloading files.
ADSL operates on copper telephone lines and can deliver faster speeds over short distances, but the signal weakens as the distance increases, leading to a limitation of around 18,000 feet.