An unknown resistor connected across a battery shows a voltage of 12 volts and a current of 4 amperes. What is the resistance?

To determine the resistance of the unknown resistor, Ohm's Law is applied, which states that resistance (R) is equal to the voltage (V) divided by the current (I). In this case, the voltage across the resistor is 12 volts and the current flowing through it is 4 amperes.

Using the formula:

R = V / I

Substituting the given values:

R = 12 volts / 4 amperes = 3 ohms

This calculation shows that the resistance of the unknown resistor is indeed 3 ohms, which corresponds to the choice labeled as option B. Understanding Ohm's Law is fundamental in solving electrical circuit problems, as it allows you to calculate any one of the three variables (voltage, current, or resistance) as long as the other two are known.