Correct Answer & Explanation
Option C
• This sentence describes an action ('sleeping') that continued for a period of time ('for two hours') in the past, up until another past action ('I called her').
• When an action continued for a duration in the past before another past action, the Past Perfect Continuous Tense is used.
• The structure for Past Perfect Continuous Tense is Subject + had been + V1-ing.
• Therefore, 'had been sleeping' is the correct form.