Final answer:
The nurse should intervene if the nursing assistant answers the phone while wearing gloves. The other actions mentioned are appropriate for providing care to a comatose patient.
Step-by-step explanation:
The nurse should intervene if the nursing assistant answers the phone while wearing gloves. This is because it is not appropriate to handle the phone or any other objects while wearing gloves, as it can lead to cross-contamination. The other actions mentioned, such as log-rolling the client to provide back care, placing an incontinence diaper, and positioning the client on the left side with the head elevated, are all appropriate and necessary for providing care to a comatose patient.