Highest surface area to volume ratio speeds diffusion Steps:

• Color change rate depends on acid diffusion, fastest for highest surface area to volume (SA/V) ratio.
• Blocks are rectangular prisms with dimensions a × b × h mm, h constant across options.
• SA/V ∝ (1/a + 1/b); calculate sums: A (1/3 + 1/30 ≈ 0.37), B (1/6 + 1/6 ≈ 0.33), C (1/6 + 1/12 ≈ 0.25), D (1/12 + 1/12 ≈ 0.17).
• A has highest SA/V, so acid reaches center quickest.

Why A is correct:

• Smallest dimension (3 mm) maximizes 1/a term in SA/V formula, speeding penetration per Fick's law of diffusion.

Why the others are wrong:

• B: Equal larger dimensions (6 mm) yield lower SA/V sum than A.
• C: 12 mm dimension lowers SA/V sum below A and B.
• D: Largest dimensions (12 mm) minimize SA/V sum.

Final answer: A