The primary therapy for secondary hypertension in children is...

The correct answer is A: Treatment of the underlying cause. Secondary hypertension in children is usually due to an underlying medical condition such as kidney disease or hormonal disorders. Treating the root cause is crucial for managing hypertension effectively. Low-salt diet (B), increased exercise (C), and weight reduction (D) are important lifestyle modifications for managing primary hypertension, not the primary therapy for secondary hypertension which requires addressing the underlying condition.