Correct Answer & Explanation
Option B
• This sentence describes two actions in the past: one ongoing action and one sudden action interrupting it.
• The action that was in progress ('washing her hair') at a specific point in the past (when her boss called) requires the Past Continuous Tense (was/were + V1-ing).
• 'Called' is in the Simple Past, indicating the interrupting action.
• Options A (Simple Past), C (Past Perfect), and D (Present Continuous) are incorrect as they do not fit the context of an ongoing past action interrupted by another past action.