If a portable tank is self-expanding, fire fighters may need to hold on to _______ while the tank is being filled.

When dealing with a self-expanding portable tank, firefighters need to hold on to the collar while the tank is being filled. The collar is typically a reinforced area around the top of the tank designed to maintain the tank's structure and prevent it from collapsing or buckling as it fills with water. Holding onto the collar provides stability, allowing firefighters to manage any potential movement or expansion of the tank material due to the water being added. This is crucial for maintaining control and ensuring the tank remains in the correct position during filling.

The other options mentioned do not offer the same level of support or relevance for managing a self-expanding tank. For instance, while the frame might provide some structure, it does not serve the specific purpose of stabilizing the expanding tank during the filling process as effectively as the collar does. The check rope is typically used for other safety measures, and the distributor refers to equipment used to control the flow of water, not to support the filling tank itself. Hence, the collar is the best choice for maintaining control during this operation.