What do parasites depend on to survive?

Parasites are organisms that live on or in a host and rely on that host for their survival and reproduction. They depend on other organisms for nutrients and a stable environment to thrive. For example, in the case of external parasites like fleas or ticks, they feed on the blood of mammals, while internal parasites like tapeworms reside in the intestines of their hosts to absorb nutrients from the host's digested food. This specific relationship highlights the interdependence between parasites and their hosts, making other organisms essential for their life cycle and sustenance.

The other options, while important in various biological contexts, do not encompass the primary need of parasites. Oxygen is necessary for many living things, moisture is vital for survival, and sunlight is crucial for photosynthesis in plants. However, these factors do not define the survival strategy of parasites, which fundamentally revolves around their relationship with living hosts.