The answer is C. virus
Any organism is said to be living if an organism has its own energy producing system and capacity to divide. Viruses do not have their own energy producing system but using energy of the host, they can make its own copy (reproduction). Making its copy is a living property. Virus can be obtained in a crystalline form like a non-living. So, viruses share the characteristics of living and non-living. Thus, virus is connecting link between living and non-living.