Final answer:
In GMA welding, the voltage is set using the voltage control knob on the welder and the amperage is set by changing the wire feed speed, with both controls typically found on the welder itself.
Step-by-step explanation:
In GMA (Gas Metal Arc) welding, the voltage and amperage settings are crucial for achieving a good quality weld. The correct answer to the question 'In GMA welding, the voltage is set on the welder, using the voltage control knob and the amperage is set on the welder, by changing wire feed speed' is option A. This is because the voltage in GMA welding is typically controlled by a dial or knob on the welder itself that allows the operator to adjust the arc length, which is directly related to the voltage. On the other hand, the amperage or current is usually set by adjusting the wire feed speed, which determines how much filler metal is fed into the weld and therefore influences the amperage.