The blank would be atender. =D Hope this helps! =D
The missing word is atender
Los chicos debe **atender **al profesor como el Sr. Rios
The missing word in the sentence is 'atender,' which means 'to attend to.' The complete sentence would be, 'Los chicos deben atender al profesor como el Sr. Rios.' This indicates that the boys should show attentiveness to the teacher.
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