Primary Parameters of Transmission Line
Transmission lines are used to transfer electrical energy from one point to another. They consist of conductors that carry the electrical signals. The primary parameters of a transmission line are the characteristics that determine its behavior and performance. These parameters are:

1. Inductance:
Inductance is the property of a transmission line that opposes any change in current flowing through it. It is caused by the magnetic field generated around the conductors. Inductance is measured in henries (H) and is denoted by the symbol L. It affects the rate of change of current and can cause voltage drops along the transmission line.

2. Capacitance:
Capacitance is the property of a transmission line that stores electrical energy in an electric field. It is caused by the voltage difference between the conductors. Capacitance is measured in farads (F) and is denoted by the symbol C. It affects the rate of change of voltage and can cause charging and discharging currents.

3. Resistance:
Resistance is the property of a transmission line that opposes the flow of electrical current. It is caused by the resistance of the conductors. Resistance is measured in ohms (Ω) and is denoted by the symbol R. It causes power losses in the form of heat and can lead to voltage drops along the transmission line.

4. Attenuation constant:
Attenuation constant is not a primary parameter of a transmission line. It is a secondary parameter that describes the rate at which the signal strength decreases along the transmission line. Attenuation constant is denoted by the symbol α and is measured in nepers per meter (Np/m) or decibels per meter (dB/m). It is influenced by both resistance and reactance of the transmission line.

Explanation:
The correct answer is option D, "Attenuation constant." The attenuation constant is not a primary parameter of a transmission line because it is not directly related to the physical characteristics of the line. It is a derived parameter that describes the loss of signal strength along the transmission line due to various factors such as resistance, capacitance, and inductance. While the attenuation constant is an important parameter in analyzing the performance of a transmission line, it is not considered a primary parameter in the traditional sense.