A phalanx is a military formation where soldiers stand side by side in close ranks, often with shields overlapping, creating a strong defensive front. This formation was famously used by Macedonian and Greek armies.
Option (B) "phagocyte" is a type of white blood cell in biology, unrelated to military formations.
Option (C) "phenomenon" means an observable occurrence or event, too general for the specific military context.
Option (D) "phaeton" is a type of carriage or car, completely irrelevant here.
Thus, "phalanx" is the correct historical and military term for the tightly packed infantry formation described.