Final answer:
Single-celled organisms similar to plants can photosynthesize. This process, exclusive to autotrophs such as plants, involves using light energy to synthesize food, distinguishing these organisms from animals.
Step-by-step explanation:
The single-celled organisms that are more similar to plants can photosynthesize. This is the ability to convert light energy into chemical energy, which is a characteristic of plants, not animals.
Photosynthesis is a process unique to autotrohs, such as plants and some single-celled organisms like algae and certain protists. These organisms possess organelles known as chloroplasts which capture sunlight and convert it, along with water and carbon dioxide, into glucose, a form of sugar that the organism uses for energy. While animals and other heterotrophs must obtain their energy by consuming other organisms, autotrophs create their own food, allowing them to sustain themselves independently.
This is why photosynthesis is the most plant-like trait and distinguishes these single-celled organisms from animals. Despite having nuclei and the capacity for movement using cilia or flagella, neither of these attributes is exclusively plant-like as they occur across various kinds of eukaryotes, including animals. Additionally, sexual reproduction is common to many eukaryotes, both plant, and animal. Therefore, the ability to photosynthesize is the definitive function that links certain single-celled organisms more closely with plants rather than animals.