Which characteristic is unique to organisms in the Plantae kingdom?

The unique characteristic of organisms in the Plantae kingdom is that they are multicellular and capable of photosynthesis. Plants are distinguished by their ability to perform photosynthesis, a process that allows them to convert sunlight, carbon dioxide, and water into glucose and oxygen, using chlorophyll contained in chloroplasts. This capability is fundamental to their survival and plays a crucial role in ecosystems, as plants are primary producers that form the base of the food chain.

Additionally, being multicellular is a defining characteristic of the Plantae kingdom. While there are some unicellular organisms that can carry out photosynthesis (such as certain algae), the majority of plants are multicellular organisms with specialized tissues and structures.

The other options highlight characteristics that do not apply to the Plantae kingdom: organisms incapable of photosynthesis, the presence of chitin in cell walls (which is found in fungi, not plants), and unicellular and heterotrophic organisms (which primarily pertain to certain bacteria and protists). These distinctions further clarify why the ability to photosynthesize and being multicellular are unique to the Plantae kingdom.