The correct answer is B) Workers aren't always available where they're needed.
The statement that best explains one of the restrictions on producers that keep the labor market from being a completely free market is that "Workers aren't always available where they're needed."
This happens because when a company needs a specifically qualified worker, probably he/she is already hired in another company and is not available at the moment. The ideal would be that when a company needs a worker, it could be found immediately as if it were a product, but it isn't. Another restriction could be labor unions that represent the interests of the workers in a negotiation.