Write
When an instrument is used for writing, we use ‘with’ before that instrument but when we mean the marks made by an instrument we use ‘in’.
Incorrect . He wrote the letter by a pen.
Correct : He wrote the letter with a pen.
Incorrect : She wrote the letter with ink.
Correct : She wrote the letter in ink.
Similarly, we can say:
He is writing in chalk on the board.
Or
He is writing with a piece of chalk.
We can also say:
The boy is writing in pencil.
Or
The boy is writing with a pencil.