Sound pressure minima occur at maximum rarefactions, where displacement slope is steepest positive.

Steps:

• Identify the displacement vs. distance graph, a sinusoidal curve showing air molecule positions.
• Locate points of maximum rarefaction, where air molecules are most spread out.
• Calculate pressure variation using ΔP ∝ -∂ξ/∂z; lowest pressure at maximum positive ∂ξ/∂z.
• Select the distance with the steepest positive slope on the graph.

Why D is correct:

• At D, the displacement curve has maximum positive slope, indicating maximum rarefaction and thus minimum ∂ξ/∂z negative, per ΔP = -B ∂ξ/∂z formula.

Why the others are wrong:

• A: Near a displacement peak, slope near zero, pressure at average.
• B: At displacement zero with shallow slope, not maximum rarefaction.
• C: Negative slope region, indicating compression, higher pressure.

Final answer: D