What is the voltage across a 3 Ω resistor if the ammeter reads 4 A?

To find the voltage across a resistor when the current is known, Ohm's Law is used, which states that voltage (V) equals the current (I) multiplied by the resistance (R). In this case, the values provided are a current of 4 A and a resistance of 3 Ω.

By applying Ohm's Law:

V = I × R
V = 4 A × 3 Ω
V = 12 V

This calculation shows that when the current of 4 A flows through a 3 Ω resistor, the voltage across it is indeed 12 V. This aligns with the correct option given, confirming that the understanding of Ohm's Law is crucial for solving voltage across resistors in electrical circuits.