The correct option to complete the sentence "Had he known he .... away" is C. would have gone. Let's break down why this is the correct choice and why the other options are not suitable. Step-by-Step Explanation

1. Understanding the Sentence Structure:
2. The phrase "Had he known" is an example of a conditional clause, specifically a third conditional. This type of conditional is used to talk about hypothetical situations in the past that did not happen. The structure typically follows the pattern:
   • If + past perfect, would have + past participle.
3. Analyzing the Correct Option (C):
4. C. would have gone: This option fits perfectly into the structure of a third conditional sentence. It implies that if he had known something (the condition), he would have taken a specific action (gone away). The use of "would have" indicates a hypothetical outcome based on a past condition that was not met.
5. Why the Other Options Are Incorrect:
6. A. will go: This option suggests a future action, which does not fit the context of a past hypothetical situation. The phrase "Had he known" indicates that we are discussing something that could have happened in the past, not something that will happen in the future.
7. B. will be going: Similar to option A, this option also refers to a future action. The phrase "will be going" implies a certainty about the future, which does not align with the conditional nature of the sentence. The sentence is about a past situation, not a future one.
8. D. may be going: This option suggests a possibility in the present or future, which again does not fit the context of the sentence. "May be going" implies uncertainty about a current or future action, rather than reflecting on a past hypothetical situation.

Summary of Key Points

• The correct answer is C. would have gone because it fits the third conditional structure, indicating a past hypothetical situation.
• The other options (A, B, and D) incorrectly suggest future actions or possibilities, which do not align with the past conditional context of the sentence.

Revision Summary

• The third conditional is used for hypothetical situations in the past: "Had he known, he would have..."
• The correct form for expressing a past hypothetical outcome is "would have" + past participle.
• Future tense options (A and B) and present/future possibility (D) do not fit the context of the sentence.
• Always identify the conditional structure to choose the correct verb form in such sentences.