Diffusion rate depends on minimum dimension

Steps:

• Acid diffuses from surfaces to center; time depends on distance to center (half the smallest dimension).
• For A (3 x 30 mm): smallest dimension 3 mm, distance 1.5 mm.
• For B (6 x 6 mm cube): smallest dimension 6 mm, distance 3 mm.
• For C/D (12 x 12 mm cubes): smallest dimension 12 mm, distance 6 mm.
• Smallest distance means fastest complete color change.

Why A is correct:

• Thinnest block has shortest diffusion path to center, per Fick's law of diffusion (rate inversely proportional to distance squared).

Why the others are wrong:

• B: Twice the diffusion distance of A, so slower color change.
• C: Four times the distance of A, much slower.
• D: Identical to C, same slow rate.

Final answer: A