Resistance is a fundamental property of a conductor that determines how much it opposes the flow of electric current. It is measured in ohms (Ω) and is denoted by the symbol R. The resistance of a conductor can be defined as the ratio of voltage to current flowing through it.

Ratio of Voltage to Current:
The resistance of a conductor is defined as the ratio of voltage to current. This means that when a voltage is applied across a conductor, the resistance determines how much current will flow through it. The greater the resistance, the smaller the current for a given voltage, and vice versa.

Mathematically, resistance (R) can be expressed using Ohm's law as:

R = V/I

Where R is the resistance, V is the voltage across the conductor, and I is the current flowing through the conductor.

Explanation of Options:
a) Ratio of voltage to current: This option is correct because resistance is defined as the ratio of voltage to current.
b) Ratio of current to voltage: This option is incorrect as resistance is not defined as the ratio of current to voltage.
c) Product of voltage and conductivity: This option is incorrect as conductivity is not directly related to resistance. Conductivity is the inverse of resistance and is a measure of how well a material conducts electricity.
d) Product of voltage and current: This option is incorrect as the product of voltage and current gives power, not resistance.

Conclusion:
The correct answer is option 'A', which states that resistance of a conductor is the ratio of voltage to current. This definition is derived from Ohm's law, which relates voltage, current, and resistance in a conductor. By understanding this concept, we can better analyze and calculate the behavior of electric circuits and the flow of current through various conductors.