Which ecosystem has the lowest level of kilocalories per square metre per year? 

A)  open ocean 
B)  tropical rain forest 
C)  lakes and streams 
D)  agricultural land

The open ocean has the lowest level of kilocalories per square metre per year among the given options. This is because the open ocean has a low primary productivity, i.e., the rate of photosynthesis by algae and plants. As a result, the energy flow in the food chain is low, leading to a low number of organisms at higher trophic levels. This, in turn, results in a low availability of food energy and hence, low kilocalories per square metre per year.