The correct option is C Root
The vegetative propagation of sweet potato can be done by roots. Sweet potato is a modification of adventitious roots, which grow from the nodes of a running stem and they are irregularly swollen due to storage of food. They can give rise to adventitious buds, which when detached can give rise to new plants thus performing the function of vegetative propagation.
Thus, the correct answer is option (C), 'Roots'.