The Fourier transform of a voltage signal x(t) is X(f). The unit of |X(f)| is

The correct answer is A. Volt.

The Fourier transform of a voltage signal $x(t)$ is $X(f)$. The unit of $|X(f)|$ is Volt. This is because the Fourier transform is a linear operator, and the unit of a linear operator is the unit of its input. In this case, the input is a voltage signal, so the output is also a voltage signal.

Option B is incorrect because Volt-sec is the unit of the time integral of a voltage signal. Option C is incorrect because Volt/sec is the unit of the derivative of a voltage signal. Option D is incorrect because Volt2 is not a unit of measurement.