Which of the following is good practice when using an oscilloscope probe?

When using an oscilloscope, particularly at high frequencies, the probe's ground lead acts like a small inductor. A long ground lead can introduce ringing, noise, and parasitic inductance into the measurement, significantly distorting the displayed waveform. Therefore, keeping the ground connection as short as possible is essential for an accurate signal representation. The other choices are incorrect. You can absolutely use a high-impedance probe on a low-impedance circuit (it actually prevents loading the circuit), and a DC-coupled probe is perfectly fine for measuring an AC circuit (it will simply show both the AC waveform and any underlying DC offset).