While walking in the woods, a student encounters a brightly colored organism with a flat, branching structure growing on a decaying branch. Back in the laboratory, she finds it contains continuous cytoplasm with no clear cell boundaries and hundreds of nuclei. This organism is most likely a .

Plasmodial slime mold

Explanation:

The description given above in the question most likely points to that of Plasmodial slime mold. Plasmodial slime mold, which is usually yellow in color, is formed by flagellated cells that swarm and merge together, forming a large, multinucleated single cell. This multinucleated single cell with a large cytoplasm have many nuclei. This slimy multinucleated single cell take the appearance of a slimy webwork structure and is commonly found on decaying logs.

The organism found growing on a decaying branch, while walking in the wood, is most likely a plasmodial slime mold.