Correct Answer & Explanation
Option B
• This sentence describes an ongoing action in the past ('repairing his car') that was interrupted or observed at a specific point in the past ('when I saw him yesterday').
• This scenario requires the Past Continuous Tense.
• The structure for Past Continuous Tense is Subject + was/were + V1-ing + Object.
• 'He' is a singular subject, so 'was repairing' is the correct form.