Final answer:
Adult T-cell leukemia/lymphoma is most commonly affected regions are Japan, West Africa, and the Caribbean.
Step-by-step explanation:
Adult T-cell leukemia/lymphoma commonly affects regions such as Japan, West Africa, and the Caribbean. For example, in Japan, this type of cancer is primarily caused by the human T-cell lymphotropic virus type 1 (HTLV-1), which is prevalent in that region. In West Africa, HTLV-1 is also a major contributing factor to the occurrence of adult T-cell leukemia/lymphoma. The Caribbean is another region where this cancer is frequently observed, again due to the high prevalence of HTLV-1.