In electric supply lines in India, the parameter that is kept constant is the frequency, which is option 3. Frequency refers to the number of complete cycles of an alternating current that occur in one second. In India, the standard frequency for electric supply is 50 Hz, meaning that there are 50 cycles of alternating current per second.

Option 1, voltage, is not kept constant in electric supply lines. The voltage can vary depending on the specific requirements of different appliances and equipment. For example, household appliances typically operate on 220-240 volts, while industrial machinery may require higher voltages.

Option 2, current, is also not kept constant in electric supply lines. The current flowing through the lines can vary depending on the power demand and the resistance of the circuit.

Option 4, power, is not a parameter that is controlled in electric supply lines. Power is the product of voltage and current, and it can vary depending on the requirements of different devices and the overall load on the system.

Therefore, the correct answer is option 3, frequency.