In the following question, select the one word which is the best substitute for the given phrase: One who is honourably discharged from service.

Introduction / Context:
This one word substitution question focuses on a term often used in academic and professional contexts. When someone leaves a position after long and honourable service, they may continue to hold an honorary title. You must recognise which word among the options captures this idea of honourable retirement, rather than warlike behaviour, absence, or working purely for money.

Given Data / Assumptions:
Phrase: One who is honourably discharged from service.
Options: Belligerent, Emeritus, Truant, Mercenary.
We look for a word typically used as a title or description for retired professionals who have served with distinction.
Common context: professors, judges or officers who retire but are still honoured with their title.

Concept / Approach:
Emeritus is a Latin derived adjective and sometimes noun, used especially for retired professors and bishops, meaning having retired but allowed to retain their title as an honour. It implies that the person left the position honourably and is respected for past service. Belligerent means warlike or aggressive; truant refers to someone who stays away from duty or school without permission; mercenary describes someone primarily motivated by money. Only emeritus matches the idea of an honourable discharge from service.

Step-by-Step Solution:
Step 1: Focus on the phrase honourably discharged from service, which suggests retirement with honour, not punishment or dismissal. Step 2: Consider Emeritus. In phrases like Professor Emeritus or Chairman Emeritus, it refers to someone who has retired from the position but keeps the title as an honour, which fits our phrase perfectly. Step 3: Consider Belligerent. Belligerent means hostile or aggressive, especially in war; it is unrelated to retirement or honourable discharge. Step 4: Consider Truant. Truant means someone who absents himself from duty or school without permission, which suggests neglect, not honour. Step 5: Consider Mercenary. Mercenary refers to someone who works only for money or a soldier hired to fight for foreign armies; again, this does not describe honourable discharge. Step 6: Therefore, Emeritus is the correct one word substitute.

Verification / Alternative check:
Look at common real world usage: many universities list retired senior scholars as Professor Emeritus or Professor Emerita. This shows that emeritus is closely associated with honourable retirement from service while retaining an honorary link with the institution. None of the other words are used in this way, confirming emergitus as the right choice.

Why Other Options Are Wrong:
Belligerent focuses on aggressiveness or a state of war and does not refer to employment status at all.
Truant is actually negative, describing someone who avoids duty or school; such a person is not honourably discharged but may be punished or dismissed.
Mercenary emphasises financial motivation rather than honour or service; a mercenary may even be viewed negatively in moral contexts.

Common Pitfalls:
Some learners may not recognise the word emeritus because it is somewhat formal and specialised. In such cases, eliminating clearly wrong options based on meaning can still lead you to the correct answer. Remember that one word substitution questions often target words that appear in news articles, university brochures and institutional descriptions, so building familiarity with such terms is useful for exams.

Final Answer:
The best substitute for the phrase is Emeritus (option B).