A gene whose expression helps to identify transformed cell is known as-
A. Structural gene
B. Selectable marker
C. Vector
D. Plasmid

Hint: Gene expression is the process by which the instructions are given to the DNA in order to convert it into a functional or useful product. For example- protein or some other important biomolecule.

Complete answer:
The selectable markers are known as the sequence of a gene, whose expression allows us to identify if the vector is introduced into the host cell or not.

A selectable marker is one of the characteristic features in a vector. They are also called the reporter gene. These markers are an antibiotic resistance gene. The genes help us to indicate whether the host cell contains the foreign DNA (gene of interest).

Let us suppose that the vector has the gene of interest. Now we want this gene of interest in huge quantities. Therefore we introduced this vector into the host cell, which here is bacteria. We want to identify whether the bacteria contain the vector nor not. So in this condition, the selectable marker plays a very important role in identifying whether transformation took place or not. As we know that selectable markers are antibiotic resistant. Now will grow the host or bacteria into the culture that has antibiotics like ampicillin. So the bacteria that have a vector (also known as the transformant cell) will grow due to the presence of antibiotic resistance ampicillin, while other bacteria (non-transformant) that do not have the vector will die.

Thus, the selectable marker helps in identifying and eliminating non-transformant and permitting the transformant to grow.
Hence, the correct answer is option (B).

Note: Transformation is the process by which the vector or a piece of DNA is introduced inside the host cell. This host cell which carries the piece of DNA is known as a transformant cell and the host which does not carry this piece of DNA is known as non-transformant.