The correct answer is option 2: single-stranded RNA.

Viruses that infect plants are typically composed of genetic material in the form of RNA, rather than DNA. This genetic material is single-stranded, meaning it consists of a single strand of nucleotides.

Option 1, single-stranded DNA, is incorrect because most plant viruses do not contain DNA as their genetic material. They primarily use RNA to replicate and infect their host plants.

Option 3, double-stranded DNA and RNA, is incorrect because plant viruses do not typically have both DNA and RNA. They usually have either RNA or DNA, but not both.

Option 4, double-stranded RNA only, is incorrect because while some plant viruses do have double-stranded RNA, it is not the most common type. Single-stranded RNA is more prevalent among plant viruses.

In summary, most viruses that infect plants possess single-stranded RNA as their genetic material.