Parallel vs. Series Resistance Formulas

Steps:

• For two identical resistors in parallel, equivalent resistance $R_{eq} = \frac{R}{2}$, where $R$ is each resistor's value.
• Given $R_{eq} = 4\Omega$, solve $\frac{R}{2} = 4$ to get .
• For series connection, $R_{eq} = R + R = 2R$.
• Substitute: $2 \times 8\Omega = 16\Omega$.

Why D is correct:

• Series formula adds resistances directly: $R + R = 2R = 16\Omega$, as current flows through both sequentially.

Why the others are wrong:

• A: 1Ω ignores formulas; it's unrelated to either configuration.
• B: 4Ω is the parallel value, not accounting for series addition.
• C: 8Ω is one resistor's value, not the total series sum.

Final answer: D