What is the effect of a negative repressible operon being in the 'on' state?

A negative repressible operon being in the 'on' state means that the operon is actively transcribing its genes. This occurs because, in a negative repressible operon, the default state is typically 'on.' The presence of a corepressor molecule is needed to activate the repressor protein, which then binds to the operator region and blocks transcription. When the operon is in the 'on' state, this repressor is not bound, allowing RNA polymerase to proceed with transcription of the genes within the operon.

The other options do not accurately describe the state of a negative repressible operon when it is actively transcribing. Being "derepressed" indicates that the operon has been turned on from an off state, while "can be silenced" suggests that there is potential for repression, which is not applicable since the operon is already active. Finally, the term "fully active" could imply more than just transcription is occurring, whereas the emphasis here is specifically on the transcription process being engaged. Thus, the description of it being 'actively transcribing' is the most precise representation of the operon's function in this state.