If the following are energy returned on investment (EROI) ratios for particular energy sources, which of the following is the most efficient?

3

10:1

48/4

2:1

48/4

Explanation:

The Energy returned on investment is the ratio among the total energy produced and the energy required to produce this final amount.

In this way the biggest the ratio, the biggest is the gain of energy, that is reason why 48/4 = 12 is the biggest ratio, that means that you gain 12 times the energy that you need to produce it.