International ohm is defined in terms of the resistance of

The correct answer is: A. A column of mercury.

The International ohm is defined as the resistance, expressed in ohms, of a column of mercury 106.3 cm long and 1 square millimeter in cross section, at the temperature of melting ice.

The other options are incorrect because:

• A cube of carbon is not used to define the International ohm.
• A cube of copper is not used to define the International ohm.
• The unit length of wire is not used to define the International ohm.