The correct option is
D Polio virus
polio or poliomyelitis is a highly contagious viral disease. It is a human enterovirus and member of the family picornaviridae. It mainly affects young children. The genome is single - stranded positive sense RNA genome and its about 7500 nucleotides. It has small genome among all organisms and hence the smallest animal virus also.
So, the correct answer is 'Polio virus'