Final answer:
The correct function to determine the number of days streetlamps can stay on based on the number of solar panels installed is D(E(n)) = 2(0.4n), which represents the composition of the energy generation function E(n) with the energy consumption function D(e).
Step-by-step explanation:
The question involves finding a function that determines the number of days streetlamps can stay on based on the number of solar panels installed. Since E(n) represents the energy generated by n solar panels, and D(e) represents the energy consumption by street lamps over e days, we need to compose these functions to get the relationship between the number of panels and the number of days. Therefore, the correct function is the composition of D and E, D(E(n)), which is D(E(n)) = 2(0.4n). This expression essentially means that for each solar panel installed, 0.4 MWh of energy is generated, and the streetlamps consume 2 MWh of energy per day.