A single-stranded, radiolabelled molecule of nucleic acids is called

The correct answer/option is probe.

Let's break down the concepts to understand why this is the correct answer:
In molecular biology, a probe (specifically a hybridization probe) is a single-stranded sequence of nucleic acid (either DNA or RNA) that is labeled with a radioactive isotope or a fluorescent tag. It is used to detect the presence of complementary nucleic acid sequences (target DNA or RNA) in a sample through the process of hybridization (pairing of complementary strands).
Let's analyze the other options to see why they are incorrect:

1. Plasmid: A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids are naturally found in bacterial cells and are commonly used as vectors in genetic engineering, but they are not single-stranded radiolabeled detection molecules.
2. Selectable marker: A selectable marker is a gene introduced into a cell, especially a bacterium or a eukaryotic cell in culture, that confers a trait suitable for artificial selection (such as antibiotic resistance). It is used to determine if a plasmid or vector has been successfully taken up by the host organism.
3. Vector: A vector is a DNA molecule used as a vehicle to artificially carry foreign genetic material into another cell, where it can be replicated and/or expressed (e.g., plasmids, bacteriophages, or viruses). They are typically double-stranded and serve a vehicle function rather than a detection function.

Therefore, a single-stranded, radiolabeled molecule of nucleic acid used for detecting complementary sequences is specifically called a probe.