This question contains the following image, and asks why Ron has a series of scars on his arms.
That's great and all, but I'm left confused;
When does Hermione get a scar reading 'Mudblood' on her arm?
She gets the scar from Bellatrix Lestrange in book 7 while she is captive at house Malfoy.
In the movie this occurs in the following scene:
According to Harry Potter Wikia:
Hermione has Mudblood written on her arm, a cut on her neck and many bites, all of them made by Bellatrix Lestrange, in 1998, at the Malfoy Manor
As Checked in the book and also as commented by Valorum:
Why shouldn’t I?” said Hermione. “Mudblood, and proud of it! I’ve got no higher position under this new order than you have, Griphook! It was me they chose to torture, back at the Malfoys!” As she spoke, she pulled aside the neck of the dressing gown to reveal the thin cut Bellatrix had made, scarlet against her throat.