balletballet says:

An essential part of your ballet uniform. White socks in a reasonably thick fabric to help you have good toes - not naughty toes!! Black and pink also available.

Manufacturer says:

Simply ballet socks which are perfect choice for dance students
Great price and excellent quality!
The sizes in order of smallest to largest are:
1. Child 6 - 8.5 (US 6.5-9.5)
2. Child 9 -12 (US 9.5-13)
3. Child 12.5 - 3.5 (US 13-5.5)
4.Adult 4-7

Fabric: 100% Nylon
Colour: Pink, black or white