Which organism requires living matter from a host to grow?

The organism that requires living matter from a host to grow is a parasite. Parasites are defined by their dependence on a host organism for their survival and reproduction. They live on or within their host, deriving nutrients and resources from the host's body. This relationship often harms the host, as the parasite may consume resources that the host needs for its own health and functioning.

To understand this in a broader context, bacteria and fungi can thrive in various environments, including dead organic matter or even artificial media, and do not necessarily require a living host for growth. Viruses, while they also need a host to replicate, are fundamentally different because they do not exhibit characteristics of living organisms until they enter a host cell. Their replication relies on hijacking the host's cellular machinery but does not involve the same symbiotic parasitic relationship that defines parasites. Thus, amongst the given options, parasites are the clear choice as they specifically need a living host to complete their life cycle.