What does "marching in formation" mean?

[Mickey]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #644 "Prepositional Phrases - In, On, Out and Under (17)", question 4

He was horrible at marching in formation. He was almost always ......... of step and responded to commands so slowly that he often ran into the soldier ahead of him while marching.

(a) in
(b) on
(c) out
(d) under

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #644 "Prepositional Phrases - In, On, Out and Under (17)", answer 4

He was horrible at marching in formation. He was almost always out of step and responded to commands so slowly that he often ran into the soldier ahead of him while marching.

Correct answer: (c) out

Your answer was: correct
_________________________

What does "marching in formation" mean?

[Mister Micawber]

Marching in straight rows and files, like THIS.
_________________
Native English teacher at Mister Micawber's