Correct

笔芯

/ bǐ xīn / 
Strokes

Example

As a Noun

pencil lead

1
Qiānbǐ
铅笔
bǐxīn
笔芯

pencil lead

2
Huàn
换
bǐxīn
笔芯

change the lead

As a Noun

refill (for a ball-pen); cartridge

3
Huàn
换
bǐxīn
笔芯

change the refill/cartridge