Young people usually have more e-------- than old people .

energy作“能力”解时通常用复数形式.
作“精力”及“能量”解时是不可数的,用单数形式.
这道题是“精力,能量”的意思.所以不能用energies.
例1.He brought new energy to the science department.
他为科学部门带来了新的能量.
例2.We hope you will call forth all your energies in doing this work.
我们希望你们做这项工作时全力以赴（能力）.