"ms. hawkins reinforces her students for each 5 minutes that they read silently. if a student stops reading during the 5 minutes, no reinforcer is earned for that time. this is an example of what schedule of reinforcement

I believe the answer is:  fixed response duration schedule

 Fixed duration schedules require the behavior be done on a consistent basis (for a set period of time). So, in order to make the behaviors keep reoccurring, the stimulus for that behavior must also be given on a fixed period of time (such as every 5 minutes like what ms. hawkin's been doing)