Pink color spreads via indicator diffusion in agar

Steps:

• Alkali solution with pink indicator is added to the agar well, starting the stopwatch.
• Pink disc forms immediately as the colored solution begins diffusing outward.
• Diffusion follows Fick's law, where distance traveled increases with square root of time, causing progressively longer intervals to reach markers.
• Observation of slowing color front confirms simple diffusion process.

Why C is correct:

• Simple diffusion is passive movement down a concentration gradient without carriers; the indicator's pink color visibly spreads this way through agar pores.

Why the others are wrong:

• A: Facilitated diffusion requires membrane proteins, absent in porous agar.
• B: Alkali ions diffuse but are colorless; pink observation tracks indicator, not alkali alone.
• D: Same as B; alkali diffusion occurs but doesn't explain visible pink spread.

Final answer: C