Final answer:
Living organisms must have energy to survive, which is fundamental for performing vital metabolic functions. This can come from sunlight or chemical compounds, underscoring energy's role as a universal requirement for all forms of life.
Step-by-step explanation:
The absolute necessity for living organisms to survive is A) energy. While other factors like sunlight, oxygen, and food in the form of other organisms are important for many life forms, none are universally required in the same way energy is. Every organism, regardless of its niche or habitat, requires energy to perform basic metabolic processes necessary for survival, such as growth, reproduction, and maintaining their internal organization.
This energy can be derived from a variety of sources, including sunlight for photosynthetic organisms, or chemical compounds in the environment for chemosynthetic organisms. The flow of energy is critical since it powers the complex interactions within ecosystems, contributing to both the biosphere and individual organism's ability to sustain life.