Final answer:
The use of bold, non-naturalistic colors is the defining feature of André Derain's London Bridge as a Fauvist painting, which focuses on conveying emotion rather than realistic representation.
Step-by-step explanation:
The feature of André Derain's painting London Bridge that is most important in its classification as a Fauvist painting is the use of bold, non-naturalistic colors. Fauvism, a movement led by artists like Derain and Matisse at the beginning of the 20th century, is characterized by its use of vibrant, exaggerated colors that often bear little resemblance to the real colors of the depicted objects.
This technique is carried out with the aim of expressing emotion, rather than accurately representing reality. In Derain's work, the Thames and London Bridge are rendered in oranges, blues, and greens that defy conventional depiction but communicate the artist's personal response to the scene.