The Colombo Plan is an initiative focusing on the development of human resources and infrastructure in member countries. Regarding cancer treatment, one specific aspect of the Colombo Plan includes efforts to enhance medical facilities and treatment capabilities.
Among the options provided, setting up cobalt therapy units is the action associated with the Colombo Plan concerning cancer treatment. Cobalt therapy is a form of radiation therapy used to treat cancer, and the establishment of such units would be in line with the infrastructure development goals of the Colombo Plan.
The other options like help with PET scan units, human resource strengthening, and setting up chemotherapy units, while important for cancer treatment, are not explicitly associated with the initiatives outlined under the Colombo Plan for cancer therapy.