What is meant by cistron?

DNA that encodes a functional product.

The correct answer is Option (4)-DNA that encodes a functional product.

The term "cistron" is an old term that was previously used to refer to a segment of DNA that encodes a single gene. The term is no longer in common use, and today the term "gene" is generally used to refer to a segment of DNA that encodes a functional product, such as a protein or RNA molecule.

In the early days of molecular genetics, researchers used the term "cistron" to describe the smallest unit of genetic function, which was thought to be a discrete segment of DNA that could be mutated and expressed as a single trait. It was later discovered that many genes are composed of multiple cistrons (or "coding regions"), each of which encodes a distinct functional domain within the gene product.