Which theory did Helmholtz propose related to color vision?

The Young-Helmholtz Theory is the correct choice because it originates from the work of both Thomas Young and Hermann von Helmholtz, who significantly contributed to our understanding of color vision. This theory posits that the human eye contains three types of photoreceptor cones, each sensitive to different wavelengths corresponding to red, green, and blue light. When these cones are stimulated in various combinations, they enable us to perceive a wide spectrum of colors. Helmholtz expanded upon Young's initial ideas, giving the theory its dual name.

Though the Trichromatic Theory is closely aligned with the core concepts of the Young-Helmholtz Theory, the latter recognizes the contributions of both scientists and provides a historical context that connects their findings. Meanwhile, the Opponent Process Theory, formulated by Ewald Hering, posits a different mechanism for color vision that involves opposing pairs of colors (e.g., red-green and blue-yellow), which adds another layer of understanding but does not pertain directly to Helmholtz's contributions. Place Theory, on the other hand, relates to auditory perception rather than visual processes, making it unrelated to the study of color vision.