HSK C1/W2
Correct

弟

/ dì / 
Strokes

Definition

younger brother/junior male/I (modest word in letter)

Example

As a Noun
younger brother
1
Xiǎodì
小弟

younger/little brother

2
Èr  
二弟

second younger brother

As a Noun
younger brother-in-law or male cousin
As a Noun
［used in old-style letters between men of the same generation, with the writer referring to himself as弟］I; me