Seaweed belongs to the group a) Protista.
Explanation: Seaweed is a type of marine algae, which are classified under the kingdom Protista. Unlike plants that are part of the kingdom Plantae, seaweeds do not have true roots, stems, or leaves. They are mainly photosynthetic organisms and play a crucial role in marine ecosystems. While some seaweeds can exhibit plant-like characteristics, they are fundamentally different from higher plants and are thus classified as protists.