In the following sentence about train timing, choose the alternative that best improves the underlined word; if no improvement is needed, select "No improvement". Do you know the time when the train departs?

Introduction / Context:
This sentence improvement question examines the use of relative and interrogative words in indirect questions. The sentence asks about knowing the time at which a train leaves, using the structure "the time when". You must decide whether this is acceptable or whether an alternative word would sound more correct and natural in standard English.

Given Data / Assumptions:

• The sentence given is "Do you know the time when the train departs?"
• The underlined word in the original exam would be "when".
• The options are "which", "by", "that", and "No improvement".
• We assume normal conversational English usage, not overly formal rewriting.

Concept / Approach:
The phrase "the time when" is perfectly natural in English to introduce a clause that specifies at what time something happens. The word "when" functions as a relative adverb in this construction. Although there are alternative ways to phrase the question, the existing wording is not wrong, so there is no need for correction. Sentence improvement questions sometimes use such items to test whether you can recognise a sentence that is already correct.

Step-by-Step Solution:
Step 1: Analyse the structure: "Do you know the time when the train departs?" The main clause is "Do you know the time", and the relative clause is "when the train departs".Step 2: Confirm that "when" correctly introduces a clause of time. It tells us at what time the train departs.Step 3: Test "which" in place of "when": "Do you know the time which the train departs?" This is ungrammatical because "which" does not function as a time adverb here and the sentence lacks a preposition.Step 4: Test "by": "Do you know the time by the train departs?" This is clearly wrong and does not make sense.Step 5: Test "that": "Do you know the time that the train departs?" This is awkward and not idiomatic, although occasionally seen; the usual pattern is either "Do you know when the train departs?" or "Do you know the time when the train departs?"Step 6: Since the original "when" is grammatically correct and natural, the best answer is "No improvement".

Verification / Alternative check:
We can compare with similar expressions: "I remember the day when we met", "He told me the year when he graduated", and "Do you know the time when the shops close?" All these sentences comfortably use "when" as a relative adverb introducing a clause of time. We could also shorten the sentence to "Do you know when the train departs?", which is even more concise, but that does not make the longer version incorrect. This confirms that no change is necessary.

Why Other Options Are Wrong:
Option A, "which", cannot replace "when" here because "which" needs a preposition like "at which" to function correctly with time and even then the wording would be overly formal.
Option B, "by", would require a very different structure and is not a correct relative word in this context.
Option C, "that", is grammatically possible in some relative clauses but sounds clumsy here and is not the standard way of expressing time.

Common Pitfalls:
Students sometimes believe that every sentence in a sentence improvement question must contain an error, so they avoid "No improvement" even when the original sentence is correct. Another common mistake is to assume that shorter or more formal sounding alternatives are automatically better, without checking whether they are actually grammatical. Always test each option carefully against standard usage.

Final Answer:
The original wording is already correct, so the answer is "No improvement", option D.