The single-phase half wave AC regulator is a circuit used to control the average power delivered to a resistive load. It consists of a diode and a variable resistor connected in series with the load. The diode allows current flow only in one direction, thus rectifying the AC voltage.

In this circuit, the load current is determined by the average voltage across the load. During the positive half-cycle of the input AC voltage, the diode is forward-biased and conducts current through the load. The variable resistor is adjusted to control the average voltage across the load, thereby regulating the load current.

Let's discuss each option to understand why option 'A' is the correct answer:

a) The rms load current is always positive:
- During the positive half-cycle of the AC voltage, the load current flows in the same direction as the voltage. As a result, the load current is positive during this interval.
- Similarly, during the negative half-cycle of the AC voltage, the diode is reverse-biased, and no current flows through the load. Hence, the load current is zero during this interval.

b) The rms load current may be positive or negative:
- This option is incorrect because the diode allows current flow in only one direction. Therefore, the load current can never be negative in this circuit.

c) The rms load current is always negative:
- This option is incorrect because the diode conducts current only during the positive half-cycle, resulting in a positive load current.

d) The rms load current can be even zero:
- This option is partially correct. The load current is indeed zero during the negative half-cycle when the diode is reverse-biased. However, the load current is positive during the positive half-cycle.

In conclusion, the correct answer is option 'A': the rms load current in a single-phase half wave AC regulator feeding a resistive load is always positive. The load current flows in the same direction as the voltage during the positive half-cycle, while it is zero during the negative half-cycle.