In the sentence "But for one witness the accused ought to have been sent to jail.", the underlined modal phrase needs to be checked. Choose the option that best improves this phrase, or select No improvement if the original is correct.

Introduction / Context:
This question tests your understanding of conditional sentences and the correct use of modal verbs in hypothetical past situations. The structure but for one witness introduces a condition that did not happen in reality. In such sentences, we normally use would have plus past participle to describe what would have happened if the condition had not existed. The phrase ought to have been sent to jail in the original could suggest moral obligation rather than a pure hypothetical result, so it needs adjustment.

Given Data / Assumptions:

• Sentence: But for one witness the accused ought to have been sent to jail.
• The underlined part is ought to have been sent.
• Options: would have, had, should have, No improvement.
• The expression but for one witness means if it had not been for one witness.

Concept / Approach:
In third conditional sentences that deal with unreal past conditions, English uses the structure would have plus past participle to express the result clause. The phrase but for plus noun is another way to introduce such an unreal condition. Therefore, the natural completion is the accused would have been sent to jail, indicating what would have happened in the past if the witness had not intervened. The original phrase ought to have been sent introduces a sense of duty or correctness rather than describing an unreal result, which does not match the but for pattern.

Step-by-Step Solution:
Step 1: Recognise that but for one witness is equivalent to if it had not been for one witness or without one witness. Step 2: Understand that this introduces an unreal past condition, which normally uses the third conditional pattern in English. Step 3: Recall the standard result form for such conditions: would have plus past participle, here would have been sent. Step 4: Replace ought to have been sent with would have been sent to show the hypothetical result correctly. Step 5: Check option A, would have, which yields the sentence But for one witness the accused would have been sent to jail. Step 6: Evaluate option B, had, which would produce the odd structure the accused had been sent to jail, not clearly tied to the conditional meaning. Step 7: Evaluate option C, should have, which introduces a sense of advisability rather than a clear unreal result and is less precise in this structure.

Verification / Alternative check:
Recast the sentence fully as a third conditional: If it had not been for one witness, the accused would have been sent to jail. This version clearly shows that the accused was not sent to jail in reality, but would have been if the witness had not intervened. The more compact form But for one witness the accused would have been sent to jail expresses the same idea. Using ought to have been sent would instead sound like the speaker is judging what should have happened, not describing a hypothetical outcome that did not occur.

Why Other Options Are Wrong:
Had by itself cannot serve as a complete modal phrase in the result clause and would make the sentence ungrammatical. Should have suggests moral or practical advice about the past rather than a pure conditional result. No improvement would leave a mismatch between the but for structure and the chosen modal, making the sentence less accurate for an unreal past condition.

Common Pitfalls:
Some learners confuse ought to have and would have in conditional sentences because both can appear with past participles. A reliable rule is that would have is the default choice for result clauses in unreal past conditions, while ought to have is reserved for expressing past duties or expectations, such as He ought to have told the truth. Recognising set patterns like but for plus would have will help you choose the right modal in exam questions.

Final Answer:
The correct improvement is would have, giving: But for one witness the accused would have been sent to jail.