The sentence requires a time clause indicating when the action of staying inside should occur. 'When it starts raining' provides the condition of time—stay inside at the time rain begins. 'Where' indicates place, not time; 'because' would require the rain to have already started; 'till it starts raining' means before the rain (which is illogical since you'd be inside before rain begins, which doesn't make sense as advice). Therefore, 'when' is the appropriate choice.
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