Final answer:
Virusoids have a small, circular single-stranded RNA genome that does not encode for any proteins and requires a helper virus for replication. They are subviral particles that show ribozyme activity and infect plants, distinguishing them from true viruses.
Step-by-step explanation:
The characteristic of virusoids that applies from the options provided is: 'Their genome doesn't encode for protein.' Virusoids are a type of subviral agent, particularly a group of single-stranded RNAs (ssRNAs), which are distinct from viruses as they do not code for any proteins. Instead, they require a helper virus to replicate. The genome of virusoids consists of a small, circular RNA molecule and is dependent on the plant cell's machinery and the associated helper virus to multiply.
Virusoids are known to infect plants, and they possess ribozyme activity, being able to catalyze certain biochemical reactions despite not encoding proteins. Notably, Group II viruses mentioned have single-stranded DNA (ssDNA) genomes and are not virusoids. Similarly, Group I and Group III viruses, with double-stranded DNA (dsDNA) and dsRNA respectively, as well as Group IV viruses with positive-strand ssRNA, are also different from virusoids.
To summarize, the non-coding nature of virusoids and their requirement for a helper virus for replication are key features that differentiate them from other viral and subviral particles.