A fight troop is incorrect. The rest are all grammatically correct.

Correct: 1. Combat Troops - Standard term for military units in combat. 2. Fighting Troops - Troops actively engaged in fighting. 3. Armed Troops - Troops equipped with weapons. 4. Troops for Fighting - Troops designated for fighting. Incorrect: 5. A Fight Troop - Use "a combat troop" or "a fighting troop" instead. Summary: * Use Combat Troops, Fighting Troops, Armed Troops, or Troops for Fighting. * Avoid A Fight Troop.