Resistance of a conductor is the ratio of voltage to current flowing through it. It is a fundamental property of any conductor and is denoted by the symbol "R". The resistance of a conductor determines how easily it allows the flow of electric current.

Explanation:

1. What is resistance?
Resistance is the property of a conductor that opposes the flow of electric current. It is caused by the collisions of free electrons with the atoms and ions in the conductor.

2. Ohm's Law:
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage applied across it, provided the temperature remains constant. Mathematically, Ohm's Law can be represented as:
V = IR
Where:
V = Voltage across the conductor
I = Current flowing through the conductor
R = Resistance of the conductor

3. Relationship between voltage, current, and resistance:
According to Ohm's Law, the resistance of a conductor can be calculated by dividing the voltage across the conductor by the current flowing through it.
R = V / I

4. Interpreting the options:
a) Ratio of voltage to current: This option is correct. Resistance is indeed the ratio of voltage to current. It represents how much voltage is required to produce a certain amount of current in a conductor.
b) Ratio of current to voltage: This option is incorrect. Resistance is not the ratio of current to voltage.
c) Product of voltage and conductivity: This option is not correct. Resistance is not the product of voltage and conductivity.
d) Product of voltage and current: This option is not correct. Resistance is not the product of voltage and current.

In conclusion, resistance of a conductor is the ratio of voltage to current flowing through it, as defined by Ohm's Law. It is an essential concept in understanding the behavior of electric circuits.