B is better because it actually tries to isolate the requested image, which is the core instruction. A describes the thumbnail well, but description is not isolation, so it misses the task despite being clearer to read.

B performs better since it makes a genuine attempt to extract the specific image that was requested, which represents the main objective of the task. While A provides a good description of the thumbnail, simply describing something differs from isolating it, so A fails to complete the assignment even though its explanation is easier to understand.