What is a plasmid?

Autonomously replicating extrachromosomal DNA.

The correct answer is Option (3) -Autonomously replicating extrachromosomal DNA.

Plasmids are small, circular, double-stranded DNA molecules that exist separately from the chromosomal DNA in bacteria and some other organisms. They replicate independently within the cell and are not considered part of the chromosomal DNA. Plasmids can carry genes that may provide advantages to the host organism, such as antibiotic resistance or the ability to metabolize certain nutrients. These characteristics make plasmids valuable tools in genetic engineering and biotechnology.