The following sentence has a blank that must be filled with an appropriate phrasal verb. Choose the correct alternative to complete the sentence: The squadron leader ordered his men to _____ immediately.

Introduction / Context:
This question tests knowledge of phrasal verbs and specific commands used in a military or parade context. The sentence describes a squadron leader giving an order to his men. The blank requires a phrasal verb that fits as a standard army drill command. Understanding such fixed phrases is important not only for vocabulary building but also for correctly interpreting instructions in reading comprehension passages about discipline or training.

Given Data / Assumptions:
The sentence is The squadron leader ordered his men to _____ immediately. The options are fall in, fall out, fall off, and fall on. We assume this refers to soldiers forming or breaking ranks on a parade ground or in a similar formal setting where standard commands are used.

Concept / Approach:
In military usage, fall in is the command used to order soldiers to form a line or to get into formation. Fall out is the opposite command, telling them to break formation and disperse. Fall off generally means to decrease in amount or to physically fall from something. Fall on often means to attack suddenly or to fall upon a surface. Since the squadron leader is giving an immediate order about assembling his men, fall in is the correct phrasal verb to complete the sentence.

Step-by-Step Solution:
Step 1: Recognise that the context is military, with a leader addressing his squadron. Step 2: Recall common parade ground commands: fall in to assemble and fall out to disperse. Step 3: Test each option in the sentence: ordered his men to fall in immediately, fall out immediately, fall off immediately, fall on immediately. Step 4: Note that fall in immediately correctly expresses the command to line up at once. Step 5: Conclude that fall in is the only phrasal verb that matches the meaning and the situation.

Verification / Alternative check:
We can verify by imagining two contrasting commands on a parade ground: The officer shouted, Fall in, and the soldiers quickly formed ranks versus After the ceremony, the officer said, Fall out, and the soldiers dispersed. This pair shows that fall in is specifically used to assemble troops. The other phrasal verbs do not appear as formal commands in this context. Therefore, using fall in in the question sentence aligns with authentic military usage.

Why Other Options Are Wrong:
Fall out is used to tell soldiers to leave formation, so it would mean the opposite of what one expects when starting an exercise or inspection.
Fall off is generally used to describe a decrease in quantity or intensity, or to describe someone physically falling from a place, not to command soldiers to line up.
Fall on usually means to attack or to land heavily on something, and it is not a standard command used by officers when addressing their troops.

Common Pitfalls:
Learners who are unfamiliar with military language may treat fall in and fall out as interchangeable or may focus only on partial meanings of fall. It is important to remember that many phrasal verbs are idiomatic and must be learned as units with specific contexts. Taking note of special phrases used in army, police, or sports settings can help in correctly answering such questions during exams.

Final Answer:
The correct phrasal verb to complete the sentence is fall in.