Q 40 ,ans in English

The complex number is $z = -i$.
In the form $x + iy$, we have $x = 0$ and $y = -1$.

Since the real part $x=0$ and the imaginary part $y=-1$, the complex number $-i$ lies on the negative imaginary axis.

For a complex number $z = x + iy$, the argument $\arg(z)$ is the angle it makes with the positive real axis.
When $z$ lies on the negative imaginary axis, the angle is $-\frac{\pi}{2}$ (or $\frac{3\pi}{2}$). The principal argument is typically in the range $(-\pi, \pi]$.
Therefore, $\text{Amp}(-i) = -\frac{\pi}{2}$.

The calculated amplitude is $-\frac{\pi}{2}$, which corresponds to option (B).

The final answer is $\text{B}$.