Here's the breakdown of the 5 levels of cell organization in a dog's body, from simplest to most complex:

1. Cells: The basic unit of life, like muscle cells, nerve cells, etc.

2. Tissues: Groups of similar cells working together, such as muscle tissue, nervous tissue, connective tissue, etc.

3. Organs: Structures made up of different tissues that perform a specific function, such as the heart, lungs, stomach, etc.

4. Organ Systems: Groups of organs working together to carry out major bodily functions, like the digestive system, circulatory system, nervous system, etc.

5. Organism: The complete living being, in this case, a dog.

So the correct order is:

Cells → Tissues → Organs → Organ Systems → Organism