An environmental agent, which triggers transcription from an operon, is a

Explanation

An environmental agent that triggers transcription from an operon is an inducer. An inducer is a molecule that binds to a repressor protein and changes its shape. This prevents the repressor protein from binding to the operator region of the operon, which allows RNA polymerase to bind to the promoter region and initiate transcription. Inducers are often the substrates of the enzymes that are encoded by the operon. This means that the operon is only turned on when the inducer is present, which ensures that the cell does not produce more of the enzyme than it needs. For example, the lac operon in E. coli is turned on by the inducer lactose. The lac operon encodes the enzymes that are needed to metabolise lactose. When lactose is present in the environment, it binds to the repressor protein and changes its shape, which allows RNA polymerase to bind to the promoter region and initiate transcription of the operon.The other options listed are not environmental agents that trigger transcription from an operon:A controlling element is a general term for any element that can regulate gene expression.Depressor is a synonym for repressor.A regulator is a general term for any protein that regulates gene expression.