Elimination of treatment types:
Hot water treatment and vapour heat treatment are disinfestation methods historically required by countries such as Japan and the USA for other fruits, but Japan's 2006 relaxation specifically required a dedicated irradiation facility to disinfest Indian mangoes of fruit flies.
Fumigation (commonly using chemicals such as methyl bromide) was the older, less preferred method being phased out internationally due to residue and environmental concerns.
By ruling out the older/alternate disinfestation routes, the treatment matching Japan's specific 2006 condition is Irradiation, using a dedicated irradiation facility set up for mango exports.