In an English sentence improvement question, choose the best alternative to replace the underlined part in the sentence: "You must complete this work up to Sunday."

Introduction / Context:
Sentence improvement questions test your ability to recognise correct preposition usage and natural phrasing in English. The sentence "You must complete this work up to Sunday" sounds awkward to a native speaker, and the task is to choose a more accurate and idiomatic alternative that expresses the idea of a deadline correctly.

Given Data / Assumptions:

• Original sentence: "You must complete this work up to Sunday."
• The underlined phrase is "up to Sunday".
• Options: within Sunday, by Sunday, on to Sunday, No improvement.
• The intended meaning is that the work must be completed not later than Sunday.

Concept / Approach:
In English, when we talk about a deadline that falls on a particular day, we generally use "by" plus the day. For example, "You must submit the form by Friday" means that the last acceptable day to submit is Friday. The phrase "up to Sunday" usually suggests continuity until Sunday, but does not clearly express a deadline for completion. Therefore, the most natural and grammatically correct preposition for this context is "by".

Step-by-Step Solution:
Step 1: Identify the function of the phrase. It is expressing a deadline for completing the work. Step 2: Recall the common structure: "complete something by a certain day". Step 3: Test option "by Sunday": "You must complete this work by Sunday." This is clear, idiomatic, and commonly used in English. Step 4: Test option "within Sunday": This sounds unnatural. "Within Sunday" is not standard usage in modern English. Step 5: Test option "on to Sunday": This is grammatically incorrect and does not convey a deadline. Step 6: Test "No improvement": This would mean "up to Sunday" is correct and natural, which it is not in standard exam English for expressing a deadline. Step 7: Conclude that "by Sunday" is the best and correct alternative.

Verification / Alternative check:
If you compare similar sentences, such as "You must pay the fee by Monday" or "Finish the project by the end of this week", you will see that "by" is consistently used to express a time limit. The phrase "up to Sunday" might be used in contexts like "The shop is open up to Sunday", but even there, speakers often prefer "until Sunday". For a deadline related to completion of work, "by Sunday" is the standard choice.

Why Other Options Are Wrong:

• within Sunday: This phrase is unnatural and rarely used. We normally say "within a week" or "within two days", not "within Sunday".
• on to Sunday: Grammatically awkward and does not express a deadline. The combination "on to" does not fit here.
• No improvement: Accepting the original phrase "up to Sunday" would preserve an awkward and non-idiomatic expression in exam-standard English.

Common Pitfalls:
Many learners are unsure which preposition to use for deadlines, and they may overuse phrases like "up to" or "within" without considering natural usage. Exam setters often exploit this confusion. A good strategy is to read plenty of authentic English materials and observe how deadlines are expressed. Remembering the pattern "by + day/date" will help you correctly answer many similar questions in sentence improvement exercises.

Final Answer:
The sentence is best improved as: "You must complete this work by Sunday."