What is the term used for a single strand of nucleic acid that is labeled with a radioactive molecule?

Probe

The correct answer is Option (3)- Probe.

Probe: A probe is a single strand of nucleic acid (DNA or RNA) that is labeled with a radioactive molecule or other types of markers (such as fluorescent dyes). Probes are used in molecular biology to detect the presence of complementary nucleic acid sequences by hybridization.

• Plasmid: A plasmid is a small, circular piece of DNA that is separate from the chromosomal DNA and can replicate independently. Plasmids are often used as vectors in genetic engineering but are not specifically labeled with radioactive molecules.
• Vector: A vector is a vehicle used to transfer genetic material into a host cell. Vectors can be plasmids, viruses, or other systems, but the term itself does not imply the presence of a radioactive label.

In summary, the correct term is probe, as it specifically refers to a single strand of nucleic acid labeled with a radioactive molecule for the purpose of detecting complementary sequences.