The correct answer is: "read only." The layers of a Docker image are read-only. Once a layer is created, it cannot be modified. Each layer represents a Dockerfile instruction, and it is stacked on top of the previous layer, forming a stack of immutable layers. These layers are designed to be read-only to ensure consistency and integrity of the image. When a Docker image is built, each instruction in the Dockerfile creates a new layer. Each layer represents the changes made by that instruction relative to the previous layer. The layers are stacked on top of each other to form the complete image. This layer-based approach allows for efficient storage and distribution of Docker images. Because the layers are read-only, any changes or modifications to the image result in the creation of new layers rather than modifying the existing ones. This immutability ensures that each layer remains intact and preserves the integrity of the image. It also enables Docker's caching mechanism, where previously built layers can be reused if the corresponding instructions haven't changed, speeding up the image build process. The other options mentioned, such as "write only," "write once," "movable," and "read mostly," do not accurately describe the permission associated with Docker image layers. Docker image layers are specifically designed to be read-only.