A single-celled organism is a complete living being that carries out all of its life processes within a single cell.

Let me explain why this is the best choice:

* Complete Living Being: Single-celled organisms are independent and capable of all the functions necessary for life (nutrition, growth, reproduction, etc.).

* All Life Processes in One Cell: Unlike multicellular organisms, they don't have specialized cells or tissues. Everything happens within the boundaries of their single cell.

Why other statements might be incorrect:

* "They are always microscopic." While many are microscopic, some single-celled organisms can be large enough to be visible to the naked eye.

* "They are only found in water." While many live in water, some thrive in soil, air, or even inside other organisms.

* "They are not as complex as multicellular organisms." While simpler in structure, single-celled organisms are incredibly complex at the molecular level and have evolved diverse strategies for survival.