Cells conserve energy and resources by making active proteins only when they are needed. If a protein is not needed, which of the following methods of control would be the most energy efficient?

The correct answer is - transcription regulation

Explanation:

Transcription regulation is a process by which a cell controls the conversion of DNA into protein-coding mRNA. Transcription regulation is important to conserve energy by making active proteins only when they are needed. It occurs both in prokaryotes and eukaryotes.

In prokaryotes, operons are found which have operator region that regulates the transcription of mRNA transcript having structural genes. Binding of repressor protein on operator inhibits the initiation of transcription.

In eukaryotes, RNA polymerase requires transcription factors and several proteins to function therefore these factors regulate gene transcription in eukaryotes. So the right answer is transcription regulation.