The correct option is B Shoot tips
An explant is any plant fragment, tissue or organ that is grown or cultured in a sterile culture medium under controlled environmental conditions via the process of tissue culture. The explant may be an excised root tip, shoot bud, leaf, petiole, anther, ovule, etc.
Growth hormones and nutrients are added to the culture media to stimulate the growth of the explant which can grow into an entire plant when subjected to varying concentrations of auxins and cytokinins.
If the parent plant is infected with virus then offspring raised from explants derived from its roots, shoot, leaves, twigs, anthers, etc have high chances of being infected as these parts might harbour the virus.
However, viruses are unable to grow within the meristematic cells of the root tips and shoot tips as these cells undergo very rapid cell division. Thus the virus is unable to complete its life cycle and multiply within these cells. Thus shoot tips or axillary buds can be used as explants to raise virus free plants via tissue culture. Such a culture using explants containing meristems is known as meristem culture.