Correct Answer & Explanation
Option C
• This sentence describes an action ('working') that started in the past and continued for a duration ('for three hours') up to another point in the past ('when I met him').
• This scenario requires the Past Perfect Continuous Tense (had been + V1-ing).
• The Past Perfect Continuous is used to emphasize the duration of an action that was in progress before another past action.
• 'Worked' (Simple Past) would imply a completed action at that point. 'Was working' (Past Continuous) would imply an ongoing action but without emphasizing the duration before the meeting. 'Has worked' (Present Perfect) is incorrect as the context is entirely in the past.