Transport number of an ion:
The transport number of an ion refers to the fraction of the total current carried by that specific ion in an electrolytic solution or a cell. It is denoted by the symbol "t" and is expressed as a decimal or fraction.

Conductance of an ion:
Conductance is a measure of an ion's ability to conduct electric current. It represents the ease with which ions move through an electrolyte solution. The conductance of an ion depends on its concentration, mobility, and charge.

Explanation:
In an electrolytic solution or a cell, multiple ions may be present. When an electric current is passed through the solution, the ions move towards the electrodes, carrying the charge. The transport number of an ion indicates the proportion of the total current that is carried by that specific ion.

The transport number of an ion can be determined experimentally using various techniques, such as the Hittorf's method or the moving boundary method. These methods allow for the measurement of changes in the concentrations of ions at the electrodes and help in calculating the transport number.

The conductance of an ion is related to its transport number. The conductance is influenced by factors such as ion concentration, ion mobility, and charge. However, it does not directly represent the fraction of total current carried by an ion.

Conclusion:
In conclusion, the fraction of the total current carried by an ion is known as the transport number of that ion. It is an important parameter in understanding the contribution of individual ions to the overall current flow in an electrolytic solution or a cell. The conductance of an ion, on the other hand, represents its ability to conduct the electric current and is influenced by various factors. Therefore, the correct answer is option 'A' - Transport number of that ion.