Final answer:
To determine how many times your web app has been unavailable during the past month, you should use option E: Service Issues.
Step-by-step explanation:
To determine how many times your web app has been unavailable during the past month, you should use option E: Service Issues. Service Issues refers to any disruptions or problems that may have occurred with the functioning of your web app. By monitoring and tracking service issues, you can determine the frequency and duration of the app's unavailability.