Final answer:
Communication trenches served as the link between front-line trenches and support trenches in the complex trench systems during World War I, particularly on the Western Front.
Step-by-step explanation:
What links the front line trenches and support trenches during World War I are the communication trenches. These were essential components of the vast trench systems used primarily on the Western Front. The communication trenches allowed the movement of soldiers, equipment, and messages between the front line, where soldiers traditionally remained for several days, and the support trenches further back, which were used as a second line of defense and for resting units.
Beyond these trenches lay even more extensive networks, including reserve trenches, long-range artillery positions, command posts, and essential infrastructure like field hospitals and logistical bases spread across the French and Belgian countryside. The notorious no-man's-land separated the opposing sides' trench systems. Despite the brutal static warfare characterized by heavy casualties for minimal territorial gains, these trenches and the links between them were meticulously maintained to ensure operational functionality and troop mobility under the direst of circumstances.