The question asks to replace the highlighted part 'He is sleeping' in the sentence 'He is sleeping when I called him.' The goal is to ensure correct tense usage, specifically when two actions occur in the past.

Grammar Rule for Past Actions

When describing two actions that happened in the past, and one action was ongoing when another action interrupted it, the following tenses are typically used:

• Past Continuous Tense for the ongoing action (the one that was interrupted).
• Simple Past Tense for the interrupting action.

Analyzing the Sentence

The original sentence is: 'He is sleeping when I called him.'

• 'He is sleeping' is in the Present Continuous Tense.
• 'when I called him' is in the Simple Past Tense.

There is a tense mismatch. The action 'is sleeping' should describe an ongoing event in the past when 'I called'. Therefore, the Present Continuous Tense ('is sleeping') is incorrect.

Evaluating the Options

• Option 1: has been sleeping - Present Perfect Continuous. This tense links past to present or describes duration, not typically used for an interruption in the past.
• Option 2: was sleeping when I called - Past Continuous ('was sleeping') correctly describes the ongoing action, and Simple Past ('called') correctly describes the interrupting action. This fits the rule.
• Option 3: is slept - Grammatically incorrect. 'Slept' is the past tense/past participle of 'sleep'. 'Is slept' is passive voice, which doesn't fit here.
• Option 4: has slept - Present Perfect. This tense connects a past action to the present, not suitable for describing an ongoing action interrupted in the past.

Conclusion

Option 2 provides the correct tense structure for the sentence. The correct sentence is: 'He was sleeping when I called him.'