The correct option is B shoot tip
Explants are small pieces of plant parts or tissues that are aseptically cut and used to initiate a culture in a nutrient medium. Generally plant shoot apical meristems are virus free. This is because the spread of the virus in the plant tissue does not keep up (is slow) vis a vis the fast meristem growth if it is actively producing new leaves/flowers. The meristem grows into a virus-free shoot that is rooted to get a virus-free plant.