A structural gene is Choose one: A. a string of nucleotides that can be decoded by an enzyme to produce a protein. B. an organism’s entire genetic complement of DNA. C. a string of nucleotides that can be used as a template to produce an RNA that codes for a protein. D. an enzyme involved in the conversion of DNA into RNA.'

C. a string of nucleotides that can be used as a template to produce a RNA that codes for a protein.

Explanation:

A gene is a specific sequence of DNA containing genetic information required to make a specific protien and  is a string of nucleotides (DNA in cells) that can be used as a template to produce a RNA (by transcription) that codes for a protein. The produced RNA might be the end product (for example some transfer RNA's) or might be further translated into a protein. An organism's entire complement of DNA is its genome.