Right choice is (a) Sectional diameter of rim on which tyre is to be mounted

For explanation I would say: The tyre size is defined as Width of tyre upon Ratio of tyre, Type of tyre, Diameter of rim to be used.

This could be understood by an example, 215/60R15 where, 215 is the width of the tyre in millimetres. 60 is the tyre profile. Radial construction is denoted by R. Diameter of the wheel rim in inches is 15.