In the following question, a sentence is given with a blank space. Choose the most appropriate preposition to complete the sentence: Mother shall return _____ an hour.

Introduction / Context:
This question checks your understanding of time-related prepositions, particularly how to express the idea of something happening a certain amount of time from now. The sentence refers to Mother's expected return and asks you to choose the correct preposition to indicate the time span of one hour in the future.

Given Data / Assumptions:
- Sentence: Mother shall return _____ an hour.
- Options: in, after, during, within.
- The intention is to say that Mother will come back one hour from now (approximately), not necessarily earlier or later than that.

Concept / Approach:
The main concept is the standard use of in with periods of time to indicate how long from now something will happen. We commonly say in an hour, in two days, in five minutes to mean a specific time interval from the present. After can sometimes work in similar contexts but often focuses on the sequence relative to an event. Within suggests any time before the end of that period, and during is used for things happening in the course of a period, not at its end.

Step-by-Step Solution:
Step 1: Interpret the target meaning: Mother will return approximately one hour from now.Step 2: Test in: Mother shall return in an hour. This is the normal, idiomatic way of expressing the idea in English.Step 3: Test after: Mother shall return after an hour. This can sometimes be used, but it suggests at some point following the completion of an hour, and is less natural in this simple prediction.Step 4: Test during: Mother shall return during an hour is ungrammatical and does not convey a specific future point in time.Step 5: Test within: Mother shall return within an hour means she will return at some time before the hour is over, which changes the meaning to an earlier time window.Step 6: Therefore, in is the best and most idiomatic preposition for this sentence.

Verification / Alternative check:
In everyday English, when people talk about an event expected a set time from now, they nearly always say in an hour, in a week, in ten minutes, etc. Saying after an hour is possible but tends to focus on the beginning of some subsequent action rather than a simple expected time. The exam-style correct answer prioritises the most common and straightforward usage, which is in an hour. The options during and within clearly do not fit the plain meaning of the sentence.

Why Other Options Are Wrong:
- After introduces a sequence after the end of a period and is not the most natural choice in this simple future expression.
- During refers to something that happens at some point within a period, not at its end or a set time from now.
- Within suggests any time before the hour is completed, which alters the meaning and is not what the sentence plainly suggests.

Common Pitfalls:
Many learners confuse in and within when talking about future times. While both relate to a time period, within emphasises a time window and can mean any time before the limit, whereas in usually indicates the approximate time at which the event will occur. To master this, remember typical phrases you hear from native speakers: I'll call you in an hour, The train will leave in ten minutes, etc. These patterns show that in is the safest and most standard choice for fixed intervals from the present.

Final Answer:
The correct preposition is in. The complete sentence is: Mother shall return in an hour.