The correct answer is B. 0.707 Imax.
The root mean square (rms) value of a sine wave is the square root of the average of the squared values of the wave. In other words, it is the value of a constant DC current that would produce the same heating effect in a resistor as the sine wave.
The rms value of a sine wave can be calculated using the following formula:
$I_{rms} = \frac{I_{peak}}{\sqrt{2}}$
where $I_{peak}$ is the peak value of the sine wave.
Therefore, the rms value of a sine wave with peak value $I_{max}$ is $I_{rms} = \frac{I_{max}}{\sqrt{2}} = 0.707 I_{max}$.
Option A is incorrect because it is the average value of a sine wave, not the rms value.
Option C is incorrect because it is the peak value of a sine wave, not the rms value.
Option D is incorrect because it is twice the rms value of a sine wave.