Parallel combinations minimize resistance; smallest resistors yield the least.

Steps:

• Use series formula: R_series = R1 + R2.
• Use parallel formula: R_parallel = (R1 × R2) / (R1 + R2).
• Compute A: (20 × 40) / (20 + 40) = 800 / 60 ≈ 13.3 Ω.
• Compute B: 20 + 100 = 120 Ω; C: (40 × 100) / (140) ≈ 28.6 Ω; D: 40 + 100 = 140 Ω.
• Compare: A has the lowest resistance.

Why A is correct:

• The parallel formula shows combining the smallest resistors (20 Ω and 40 Ω) produces the minimal equivalent resistance, as smaller values increase the reciprocal sum.

Why the others are wrong:

• B: Series addition yields a large total (120 Ω), far exceeding parallels.
• C: Parallel of larger resistors (40 Ω and 100 Ω) gives higher resistance (≈28.6 Ω) than A.
• D: Series of larger resistors results in the highest value (140 Ω).

Final answer: A