The correct answer is D. Feed.

All living organisms require a source of energy to sustain their life processes, and they obtain this energy by consuming other organisms or organic matter. Photosynthesis is a process by which some organisms, such as plants, produce their own food using sunlight, but not all living organisms have the ability to do so. Respiration is a metabolic process that occurs in all living cells, but it is not exclusive to living organisms, as non-living cells can also undergo respiration. Movement is a characteristic of many living organisms, but not all, for example, plants may not move in the same way as animals. Therefore, feeding is the most fundamental characteristic of all living organisms.

Option B (Respire) is wrong : Respiration is indeed a characteristic of all living organisms, but it is not exclusive to them. Non-living cells can also undergo respiration, for example, yeast cells used in baking or wine-making. However, feeding is a characteristic that is exclusive to living organisms. Therefore, feeding is considered the most fundamental characteristic of all living organisms.