Explanation:

This question requires us to choose the most appropriate modal verb to express a past obligation or duty, indicated by the context '(Obligation)' and the supporting statement 'He is the only friend you have.' The structure 'modal + have + past participle' is used to refer to past actions. **Understanding the Context:** The sentence implies that the person *should have* helped their friend in the past, but perhaps didn't, and there's a sense of duty or strong recommendation associated with that unfulfilled action. The phrase 'He is the only friend you have' underscores the moral imperative or strong expectation of help. Let's analyze each option: **Option (A): might have helped** 'Might have + past participle' expresses a possibility in the past, or a weaker degree of certainty. For example, 'He might have helped, but I'm not sure.' It does not convey a strong sense of obligation or duty. While helping was a possibility, this modal doesn't capture the 'obligation' aspect emphasized in the question. **Option (B): ought to have helped** 'Ought to have + past participle' is used to express an unfulfilled past obligation, a strong recommendation, or a moral duty that was not carried out. It conveys that something should have been done in the past but wasn't. This perfectly aligns with the 'Obligation' specified in the question and the sentiment conveyed by 'He is the only friend you have,' suggesting a duty that should have been fulfilled. For example, 'You ought to have studied harder' implies a missed opportunity or duty. This is the most appropriate modal for the given context. **Option (C): would have helped** 'Would have + past participle' is typically used in the main clause of a third conditional sentence (e.g., 'If you had asked, I would have helped.') to describe a hypothetical past action that did not happen. It can also express a past habit or willingness, but not a direct obligation or duty in this standalone context. It doesn't convey the sense of 'should have' that the context demands. **Option (D): will have helped** 'Will have + past participle' forms the future perfect tense, which describes an action that will be completed by a certain point in the future. For example, 'By next year, I will have helped many people.' This tense is entirely inappropriate for expressing a past obligation. It refers to future completion, not past duty. **Conclusion:** Given that the sentence expresses a past obligation or a strong sense of duty that was likely unfulfilled, 'ought to have helped' is the most fitting modal construction. It directly addresses the 'Obligation' requirement. Therefore, option (B) is the correct answer.