Option 1: stood off -> means to keep at a distance or remain aloof, which doesn't fit the context of being captivating.
Option 2: stood out -> means to be prominently noticeable or distinctive, perfectly describing a painting that catches attention.
Option 3: stood by -> means to support someone or wait nearby, which is irrelevant for describing a painting's visual impact.
Option 4: stood over -> means to watch someone closely or loom over, which doesn't apply to how a painting appears in a gallery.
Hence, Option 2: stood out -> This phrasal verb perfectly captures how the vibrant painting was prominently noticeable and eye-catching among other artworks, which aligns with it captivating viewers with its bold colors and unique style. -> correct