Diffusion rate depends on thinnest dimension and exposed surface area Steps:

• Identify thinnest dimension for each block, as acid diffuses shortest path to center there.
• Compute half thinnest dimension: time ∝ (distance)^2 from diffusion equation.
• For flat shapes, large exposed faces accelerate influx via higher surface area.
• Compare: smallest effective distance with maximal area yields fastest complete change.

Why B is correct:

• Thinnest dimension 1 mm (distance 0.5 mm) with flat shape and large 2.5 mm × 2.5 mm faces maximize acid influx per Fick's first law (flux = -D × area × ∇c).

Why the others are wrong:

• A: Same 1 mm thickness but cubic shape limits exposed area, reducing influx rate.
• C: Thinnest 2 mm (distance 1 mm) doubles squared time, slowing diffusion.
• D: Thin 0.5 mm dimensions but elongated 2 mm length delays acid reaching far interior along axis.

Final answer: B