Which part would be most suitable for raising virus-free plants for...

Generally, it is believed that actively growing regions of plants (i.e., shoot apex and root apex) do not have viruses. Due to this big advantage, these regions are used in the production of virus free plants through meristem culture technique. In meristem culture explants are taken from shoot apex or shoot apical meristem. These explants are cultured on a medium containing more cytokinin than auxin hormone. Thus, meristem culture involves the development of an already existing shoot apical meristem and subsequently, the regeneration of adventitious root from the developed shoots.