In the following question, select the antonym (opposite in meaning) of the word "vigour".

Introduction / Context:
This vocabulary question asks you to find the antonym of "vigour". "Vigour" is a common English noun used in both general and academic contexts. Knowing its meaning and typical opposites is important for synonym antonym based questions in competitive exams.

Given Data / Assumptions:

• Target word: "vigour".
• Options: "lethargy", "fettle", "kilter", "robust", "fitness".
• We must choose the word that expresses the opposite meaning.
• "Vigour" usually refers to energy, strength, or active force.

Concept / Approach:
"Vigour" means physical strength, good health, or high energy, and sometimes intellectual or emotional force. Its opposite would therefore be a state of tiredness, lack of energy, or sluggishness. "Lethargy" perfectly captures this opposite meaning: a state of drowsiness, inactivity, or lack of enthusiasm. The other options all express positive states of health or good condition and are closer to synonyms, not antonyms.

Step-by-Step Solution:
Step 1: Define "vigour" as energy, vitality, and strong physical or mental force.Step 2: Look among the options for a term that means the opposite—low energy, tiredness, or sluggishness.Step 3: Recognise that "lethargy" means a state of inactivity, sleepiness, or lack of interest.Step 4: Notice that "fettle", "kilter", "robust", and "fitness" all suggest good condition or strength, which are similar in meaning to "vigour".Step 5: Therefore select "lethargy" as the antonym.

Verification / Alternative check:
Consider the sentence: "He tackled his work with great vigour." Replacing "vigour" with "lethargy" reverses the meaning: "He tackled his work with great lethargy" would mean he acted very slowly and lazily. This confirms that "lethargy" is the opposite of "vigour". The other terms, such as "robust" or "fitness", would not produce an opposite meaning in such a sentence.

Why Other Options Are Wrong:

• fettle: In the phrase "in fine fettle", it means good health or good condition, which is similar to "vigour", not opposite.
• kilter: In the phrase "in good kilter" or "out of kilter", it refers to proper condition or balance; by itself it is not an antonym of "vigour".
• robust: Means strong and healthy, clearly close to "vigorous".
• fitness: Means the state of being physically fit and healthy, also related to "vigour".

Common Pitfalls:
Some examinees may not know less common words like "fettle" and "kilter" and might assume that unknown words must be correct. However, both these words are actually associated with good condition, not tiredness. When in doubt, rely on what you definitely know: "lethargy" is widely used to describe low energy, making it the safest and correct choice here.

Final Answer:
The antonym of "vigour" is lethargy.