What is needed by every living organism for existence?

The correct answer is water. Every living organism requires water for survival because it is essential for various biological processes. Water serves as a solvent in which many of the body's chemical reactions take place. It helps transport nutrients and waste products, regulates body temperature through sweating and respiration, and participates directly in metabolic activities.

While oxygen, non-biological substances, and sunlight are important for certain organisms or processes, they do not apply universally to all living organisms. For example, some organisms, like certain anaerobic bacteria, can survive without oxygen. Non-biological substances, such as minerals, are critical for specific functions but are not fundamentally required for the existence of all organisms. Sunlight is vital for photosynthetic organisms, but many organisms, including those in dark environments like deep sea ecosystems, thrive without it.

Thus, water is the most universally necessary substance for the existence of all living organisms, making it the correct answer.