The correct answer is B. non-linear.
A rectifier instrument is an instrument that measures the average value of an alternating current or voltage. The scale of a rectifier instrument is non-linear because the average value of an alternating current or voltage is not proportional to the peak value.
The average value of an alternating current or voltage can be calculated using the following formula:
$V_{avg} = \frac{1}{T} \int_0^T V(t) dt$
where $V(t)$ is the instantaneous value of the alternating current or voltage, $T$ is the period of the alternating current or voltage, and the integration is performed over one period.
The peak value of an alternating current or voltage can be calculated using the following formula:
$V_{peak} = V_{max} – V_{min}$
where $V_{max}$ is the maximum value of the alternating current or voltage and $V_{min}$ is the minimum value of the alternating current or voltage.
As you can see from the formulas, the average value of an alternating current or voltage is not proportional to the peak value. This is why the scale of a rectifier instrument is non-linear.