The correct answer for the question is option 1, which states that Ohm`s law defines resistance. Ohm`s law is a fundamental law in electrical circuits that relates the voltage across a conductor to the current flowing through it. According to Ohm`s law, the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with the constant of proportionality being the resistance (R) of the conductor. Mathematically, Ohm`s law can be expressed as V = I * R.

Although Ohm`s law does involve the concepts of current and voltage, it is specifically concerned with the relationship between these two quantities and the resistance of a conductor. Option 2, which states that Ohm`s law defines current only, is incorrect as it does not acknowledge the role of resistance. Similarly, option 3, which states that Ohm`s law defines voltage only, is incorrect for the same reason. Option 4, which states that Ohm`s law defines both current and voltage, is also incorrect because it fails to mention resistance, which is the primary factor defined by Ohm`s law. Therefore, option 1 is the correct answer.