Ohm's Law: Voltage across fixed resistance is directly proportional to current

Steps:

• Voltmeter measures voltage V across the fixed coil resistance R.
• Ammeter measures current I through the series circuit including variable resistor.
• Varying the variable resistor changes total resistance and thus I.
• By Ohm's law, V = I × R, yielding a linear V-I relationship with slope R.

Why C is correct:

• Graph C is a straight line through the origin, matching V = I R where R = 3.0 V / 0.50 A = 6 Ω.

Why the others are wrong:

• A shows constant V (independent of I), ignoring resistance effect.
• B shows hyperbolic curve (like 1/I), not linear proportionality.
• D shows upward curve (nonlinear), violating constant R.

Final answer: C