Q.Speed of sound is maximum in (A) air (B) water (C) vacuum (D) solid

[!TLDR] Solids have far larger elastic moduli than liquids or gases, so sound (needing elastic restoring force) travels fastest through them; …

Sound is a longitudinal mechanical wave whose speed is $v=\sqrt{E/\rho}$ (or the Newton-Laplace form for gases) -- it depends on the medium's elastic modulus E and density ρ. Solids resist compression FAR more strongly than liquids, which in turn resist it more strongly than gases, i.e. $E_{solid}\gg E_{liquid}\gg E_{gas}$, and this dominates over any differences in density, so sound consistently travels fastest through solids. Table 8.1 confirms this numerically: air is only ~331-343 m/s, water ~1400-1500 m/s, while solids like copper, steel, granite and aluminium all exceed 3500 m/s (aluminium reaching 6420 m/s). Vacuum, having no medium at all, …

Compare typical elastic moduli of solids, liquids and gases (solids largest) using $v=\sqrt{E/\rho}$, and rememb …

Forgetting that vacuum is not simply 'the slowest' option but an outright impossible medium for sound (a longitudinal MEC …