The impedance of the equipment grounding conductor can only be accurately measured with an instrument known as a(n):

A volt ohm meter measures DC resistance and AC/DC voltage, but cannot accurately measure the AC impedance of a grounding conductor under the conditions required by grounding standards.

A power distribution unit is electrical distribution equipment used to supply power to devices, not a measurement instrument of any kind.

An exothermic weld connection is a method for permanently joining grounding conductors using a chemical exothermic reaction, not an instrument for measurement.

A ground impedance tester is purpose-built to measure AC impedance of grounding conductors and grounding electrode systems under controlled test conditions - standard multimeters measure only DC resistance and cannot characterize the AC impedance behavior of a grounding path. Accurate impedance measurement is critical because a low DC resistance does not guarantee low AC impedance, which is what matters for effective fault current dissipation.

An amp calculator is not a recognized test instrument - current measurement on a grounding conductor would use a clamp meter, but even that does not measure impedance.