Explanation:
The positive sequence current of a transmission line refers to the current that flows in all three phases of the transmission line in the same direction and with the same magnitude. In other words, it is the current that is in phase with the voltage and is responsible for the real power transfer.

On the other hand, the negative sequence current of a transmission line refers to the current that flows in all three phases of the transmission line in the opposite direction to the positive sequence current and with the same magnitude. It is responsible for the generation of magnetic fields and does not contribute to the real power transfer.

Now, let's consider the options given in the question:

a) always zero - This is not true as the positive sequence current can be non-zero in normal operating conditions.

b) 1/3 of negative sequence current - This is not true as the positive sequence current and negative sequence current are not related in this way.

c) equal to negative current - This is the correct option as the positive sequence current and negative sequence current are equal in magnitude but flow in opposite directions.

d) 3 times negative sequence current - This is not true as the positive sequence current and negative sequence current are equal in magnitude.

Therefore, option 'C' is the correct answer.

In summary:

- The positive sequence current flows in all three phases of the transmission line in the same direction and with the same magnitude.
- The negative sequence current flows in all three phases of the transmission line in the opposite direction to the positive sequence current and with the same magnitude.
- The positive sequence current is responsible for real power transfer while the negative sequence current generates magnetic fields.
- The positive sequence current and negative sequence current are equal in magnitude but flow in opposite directions.