HSK C7-9
Correct

函

/ hán / 
Strokes

Definition

envelope/case/letter

Example

As a Noun
case; envelope; casket
1
Shíhán
石函

stone casket

2
Jìnghán
镜函

case for a mirror

3
Jiànhán
剑函

case for a sword

As a Noun
letter
4
Zhìhán
致函

send/write a letter

5
Suǒpéi
索赔
hán
函

letter of claim