The correct answer is observation of cues.
Observation of cues refers to a communication strategy used by co-teachers, wherein they communicate with each other non-verbally. Co-teachers might come up with communication signals, such as hand gestures, facial expressions, code words, etc., to quickly convey short messages to each other. Non-verbal communication between co-teachers is important, since it is not always appropriate or possible to discuss students' performance during classroom hours. In this instance, when the circulating teacher holds up one finger when a student's work illustrates comprehension of the skill and two fingers up when a student's work does not illustrate comprehension, she is using observation of cues.