In vocabulary and everyday English, which single word best expresses the meaning ‘‘a duty or task that must be fulfilled or carried out’’?

Introduction / Context:
Verbal ability and English vocabulary sections often include one word substitution questions, in which a phrase must be replaced by a single precise word. The phrase given here describes something that a person is required or expected to do, either morally, legally, or by agreement. Identifying the correct word improves both exam performance and everyday language skills.

Given Data / Assumptions:

• The phrase is a duty or task to be fulfilled.
• We must find a single word with this meaning.
• Options include obligation and several unrelated words that share the same ending.
• We assume standard modern English usage.

Concept / Approach:
An obligation is something a person is bound to do, whether because of a promise, a law, a contract, or moral responsibility. It can refer to duties at work, responsibilities towards family, or commitments to fulfil agreements. The other options are unrelated nouns that do not involve the idea of required duty or task. Therefore, obligation is the clear one word substitution for the phrase in the question.

Step-by-Step Solution:
Step 1: Focus on the key ideas in the phrase duty and to be fulfilled, which imply responsibility and necessity.Step 2: Recall that obligation is commonly used when talking about legal or moral duties, such as financial obligations or social obligations.Step 3: Compare this with option A, which exactly matches the meaning of a task one is bound to perform.Step 4: Check the other options. Conversation refers to talking, decoration to ornamentation, navigation to guiding a vehicle, and imagination to creative thinking.Step 5: Since none of the other options express the idea of duty, obligation must be the correct answer.

Verification / Alternative check:
Dictionaries define obligation as something that a person is bound to do, especially by law, duty, or promise. Example sentences include phrases like having an obligation to pay debts or feeling an obligation to help friends. These definitions closely match the phrase used in the question, reinforcing that obligation is the right one word substitute.

Why Other Options Are Wrong:
Option B, conversation, relates only to speaking and listening, not to duty. Option C, decoration, refers to adding ornamental elements to objects or spaces. Option D, navigation, deals with directing ships, aircraft, or vehicles along a route. Option E, imagination, is about forming new ideas or images in the mind. None of these capture the sense of required responsibility described in the phrase a duty or task to be fulfilled.

Common Pitfalls:
Because the options all end with the same suffix tion, some candidates may be tempted to guess based on sound rather than meaning. Another pitfall is to overthink the context and look for a more complex legal term. Remember that verbal ability questions often check straightforward vocabulary, and recognising common words like obligation is usually sufficient.

Final Answer:
The single word that best expresses the idea of a duty or task to be fulfilled is Obligation.