Thermal conductivity is a measure of a material's ability to conduct heat. Materials with lower thermal conductivity are better insulators.

Thermal conductivity is a measure of a material's ability to conduct heat. Materials with lower thermal conductivity are better insulators.

Let's compare the typical thermal conductivities of the given substances:

• Air: Air is a gas. Gases generally have very low thermal conductivity because their molecules are far apart, leading to fewer collisions and less efficient heat transfer by conduction. The thermal conductivity of still air is approximately $0.024 \text{ W/(m}\cdot\text{K)}$.
• Water: Water is a liquid. Liquids typically have higher thermal conductivity than gases but lower than most solids. The thermal conductivity of water is approximately $0.6 \text{ W/(m}\cdot\text{K)}$.
• Ash: Ash is a powdered solid. Its insulating properties come largely from the air trapped within its porous structure. While ash is a good insulator, its effective thermal conductivity (which includes the solid particles and trapped air) is typically higher than that of pure still air, often in the range of $0.07 \text{ to } 0.15 \text{ W/(m}\cdot\text{K)}$.
• Glass: Glass is a solid. Solids generally have higher thermal conductivity than liquids and gases. The thermal conductivity of glass is approximately $0.9 \text{ to } 1.3 \text{ W/(m}\cdot\text{K)}$.

Comparing these approximate values:

• Air: $0.024 \text{ W/(m}\cdot\text{K)}$
• Ash: $0.07 - 0.15 \text{ W/(m}\cdot\text{K)}$
• Water: $0.6 \text{ W/(m}\cdot\text{K)}$
• Glass: $0.9 - 1.3 \text{ W/(m}\cdot\text{K)}$

Based on these values, air has the least thermal conductivity among the options.

The final answer is $\text{B) air}$.