Final answer:
The most appropriate therapy for this child is oxygen therapy to improve their low Spo2 level and alleviate their symptoms.
Step-by-step explanation:
The therapy most appropriate for this child would be oxygen therapy, as indicated by the low Spo2 level of 85%. The child is experiencing symptoms such as poor feeding, irritability, lethargy, labored breathing, rapid pulse, dusky color, and weak/absent pulse. These symptoms, along with the low Spo2 level and vital signs indicating respiratory distress, suggest that the child is not getting enough oxygen and needs supplemental oxygen. By providing oxygen, the child's Spo2 level will improve, which can help alleviate their symptoms and improve their overall condition.