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The correct choice for replacing a 300 Ohm resistor is based on understanding how resistors behave in series and parallel configurations. A resistor is considered as part of a circuit that determines the overall resistance based on how the resistors are arranged.
When resistors are connected in series, their resistances add up. This means that two 150 Ohm resistors in series would result in a combined resistance of 300 Ohms (150 + 150 = 300). Therefore, this option is viable for replacing a 300 Ohm resistor.
On the other hand, connecting two 600 Ohm resistors in parallel results in a lower equivalent resistance than either resistor alone. The formula for two resistors in parallel is given by 1/R_total = 1/R1 + 1/R2. If you were to calculate, two 600 Ohm resistors in parallel would yield an equivalent resistance of 300 Ohms, but that is not a viable arrangement if a standard resistor is needed that directly equals 300 Ohms without introducing additional complexities in the circuit.
Replacing the burnt-out 300 Ohm resistor with either one 300 Ohm resistor or two 150 Ohm resistors in series retains the same effective resistance in the circuit