Which painter is associated with the theory of realism and nature advocated by John Ruskin?

John Everett Millais is associated with the theory of realism and nature advocated by John Ruskin due to his involvement in the Pre-Raphaelite Brotherhood, which he co-founded. This group aimed to return to the detailed observation of nature and to capture the authentic beauty found in everyday life, reflecting Ruskin's emphasis on the importance of realism in art.

Millais' work often embodied the principles that Ruskin championed, including a focus on detailed naturalism and an emotional connection with the subjects portrayed. The Pre-Raphaelites sought to challenge the existing standards of art by prioritizing direct observation and a truthful representation of the natural world, aligning closely with Ruskin’s viewpoints regarding art's moral and social responsibilities.

In contrast, the other artists mentioned represent different movements or styles. Vincent van Gogh is known for his post-impressionistic style, which, while it includes elements of nature, does not strictly adhere to the principles of realism as set forth by Ruskin. Henri Rousseau is known for his naive style and imaginative landscapes rather than realism. Jackson Pollock is a prominent figure in abstract expressionism, which moved away from representational art entirely, focusing instead on the expressive potential of painting itself.