I would say the correct answer is B: People tend to overuse or degrade shared resources. It is often not clear how much exploitative pressure a natural resource can sustain before the exploitation becomes unsustainable. So for example, various industries may dispose of waste into a river. The river has a capacity to absorb and clean a certain amount of effluent by natural degradation systems. However, at some critical mass of pollutants, these natural processes are no longer able to function properly, and the river becomes very polluted to a point were the resource cannot be utilised. Monitoring can help to gauge at which point a common resources is becoming unacceptably degraded, and this can guide policy.