Which type of cabbage is this?

Bok choy (American English), pak choi (British English), or pok choi (Brassica rapa subsp. chinensis) is a type of Chinese cabbage. Chinensis varieties do not form heads and have green leaf blades with lighter bulbous bottoms instead, forming a cluster reminiscent of mustard greens. Chinensis varieties are popular in southern China and Southeast Asia. Being winter-hardy, they are increasingly grown in Northern Europe.

Now considered a subspecies of Brassica rapa, this group was originally classified as its own species under the name Brassica chinensis by Carl Linnaeus. They are a member of the family of Brassicaceae or Cruciferae, also commonly known as the mustards, the crucifers, or the cabbage family.