Correct

低能

/ dīnéng / 
Strokes

Definition

incapable/incompetent/stupid/mentally deficient

Example

As an Adjective

mentally retarded/deficient; feeble-minded; imbecile; idiotic

1
Dīnéng
低能
bānjí
班级

class of low intelligence

As an Adjective

low-energy

2
Dīnéng
低能
zhàomíng
照明
shèbèi
设备

low-energy lighting system