Conditional Sentences

Let's analyze the options:

• knew, go: This pair uses the simple past and base form, which doesn't fit the past unreal condition (type 3 conditional) context.
• had known, have visited: This uses the past perfect "had known" and the perfect infinitive "have visited," matching the type 3 conditional structure.
• have known, be going: This pair mixes present perfect and base form, which does not align with past conditional structures.
• know, had gone: This pair uses the present simple and past perfect forms, which is also incorrect for past unreal conditions.

Therefore, the correct completion is: "If I had known, I would have visited him." This perfectly fits the third conditional structure, indicating a past unreal condition and its hypothetical result.