What is the characteristic of parasites?

Parasites are organisms that live on or inside a host organism and rely on that host for essential nutrients and shelter to survive. Unlike other organisms that can thrive independently in their environments, parasites specifically depend on their hosts for their life processes, including feeding and reproduction. This relationship often harms the host, which can suffer from various effects such as nutrient depletion or disease.

In contrast to the other choices, which suggest independence or beneficial relationships, the defining feature of parasites is their dependence on a host for survival. This unique characteristic fundamentally distinguishes parasites from free-living organisms or mutualistic symbionts, which can either lead to independent survival or benefit their hosts.