Which energy level has the least energy?
n=1
n=3
n=5
n=7

n = 7

Explanation:

The Energy of a system decrease through the levels. Is simple to understand that:

1. All energy cames from sun.

2. Organisms that can absorve that energy acumulates X quantity. The more comum way to do that is the photosynthesis, which turns solar energy into glucose, basically. (Equation of photosynthesis: 6H2O + 6CO2 = C6H12O6 + 6O2)

3. The first heterotrofic organism eats a plant, for example. Then he gets a part of that energy, aproximately 0,3X - the efficiently variates for each organism, but it's low. For example, we need to took 30 Kcal to use 100 Kcal from a common protein. And we need to lose more energy for every process that we will do in our body. Remember, or now know, the second law of termodinamics: is impossible to fully convert the ceded energy into WORK in a open system.

4. The next organism eats the first, getting a part of his energy. 0,3 x 0,3X

So, as this process repeats for all organisms, we can easily conclude that the last organism has the least energy.