Which type of organism requires a living host for survival?

The correct response is that parasites require a living host for survival. Parasites are organisms that live on or in a host organism and derive nutrients at the host's expense. They rely on their hosts for food, shelter, and means of reproduction. This dependence on a host is essential for their lifecycle and sustainability, making them unique among organisms.

In contrast, while bacteria, viruses, and fungi can have varied lifestyles and some can thrive in non-living environments, parasites are specifically adapted to depend on their living hosts. Bacteria can live independently in various environments, viruses need a host cell to replicate but do not consistently live within a host as a parasitic organism does, and fungi can grow in decaying matter or other non-living substrates, not strictly requiring a living host for their existence. Thus, the distinctive characteristic of parasites is their reliance on a living host for survival and reproduction.