Final answer:
The correct completion of the sentence using the third conditional is option c) 'I would not have missed the train.' The third conditional is used to talk about imaginary or unreal past situations and their hypothetical outcomes.
Step-by-step explanation:
The correct completion of the sentence using the third conditional is option c) I would not have missed the train.
The third conditional is used to talk about imaginary or unreal past situations and their hypothetical outcomes. In this case, the student is expressing regret about not being earlier and the consequence is missing the train. By using 'would' + 'have' + the past participle of the main verb ('missed'), option c) correctly completes the sentence.
For example: 'If I had been earlier, I would not have missed the train.' This sentence expresses the idea that in the unreal past when the student wasn't earlier, the consequence of missing the train could have been avoided.
The sentence "If I had been earlier," should be completed using the third conditional, which is used to talk about past situations that did not happen and their hypothetical results. The correct completion of this third conditional sentence is option (c) "I would not have missed the train." The third conditional is formed using if + past perfect, and the main clause with would + have + past participle.