What is the primary characteristic of viruses?

The primary characteristic of viruses is that they need a host to grow. Viruses cannot replicate or carry out metabolic processes on their own; they lack the cellular machinery that living organisms possess. Instead, they rely on invading a host cell, where they hijack the host’s cellular mechanisms to reproduce and make copies of themselves. This dependency on a host for reproduction and survival is a defining feature that distinguishes viruses from living organisms, which can carry out these processes independently.

The other options suggest capabilities or characteristics that do not align with the nature of viruses: they do not possess the ability to survive independently, cannot reproduce autonomously, and are not considered complex organisms on their own since they are essentially composed of genetic material encased in a protein coat.