Which plant is traditionally known for its use as "willow bark"?

Willow bark is traditionally recognized for its medicinal properties, primarily due to its content of salicin, a chemical precursor to salicylic acid, which is the active component in aspirin. Salix alba, commonly known as white willow, is the species specifically associated with the use of its bark in traditional medicine to relieve pain and reduce fever. The historical significance of willow bark reflects its long-standing application in herbal remedies, demonstrating its effectiveness as a natural analgesic and anti-inflammatory agent.

Other plants listed do have their own medicinal uses, but they do not bear the same historical relevance to the term "willow bark." Achillea millefolium, or yarrow, is known for its wound-healing properties; Mentha piperita, or peppermint, is mainly used for digestive issues and flavoring; and Pinus sylvestris, or Scots pine, is recognized for its timber and essential oils rather than any association with willow bark. Thus, Salix alba distinctly stands out as the correct answer related to the specific use of "willow bark."