The basic characteristic of a living organism is the ability to breathe. Breathing is the process of taking in oxygen and releasing carbon dioxide. It is essential for the survival of living organisms as it allows them to obtain the necessary oxygen for cellular respiration and remove waste products. Option 2, the ability to reproduce, is also a characteristic of living organisms, as it allows them to pass on their genetic material to the next generation. Option 3, the ability to move, is not exclusive to living organisms, as some non-living things can also move. Option 4, the ability to eat, is important for obtaining nutrients, but it is not the essential characteristic that defines a living organism. Therefore, the correct answer is option 1.