What information about the LED circuit is required to calculate which resistance value should be used?
A. Source voltage and LED maximum forward current (50%)
B. Wavelength of light, LED forward voltage, and LED maximum forward current (2%)
C. Source voltage, LED forward voltage, and LED maximum forward current (23%)
D. LED forward voltage and LED maximum forward current (24%)

Correct answer is C because according to UWorld, "By conservation of energy, the voltage rise from the voltage source VS is equal to the sum of the voltage drops across the other elements in the circuit, the resistor VR and the LED VF," and so both Vr and Vf are needed. But in the passage, it states, "The LED itself does not behave like a resistor: When current flows through the LED, the LED voltage remains constant, known as the forward voltage."
Therefore, there should be no voltage drop over the LED, so Vf shouldn't be needed to get the total resistance in the circuit. What gives?

The passage states that "when current flows through the LED, the LED voltage remains constant", meaning that the forward voltage remains constant within a wide range of changing currents, compared to how a resistor changes its voltage due to a changed current (V=IR). It does not mean that the Vf is nonexistant. I think the passage is playing mind games by phrasing it differently to confuse students who don't have a grasp of sciences behind it (or who solely relied on the passage).