Common materials around us such as water, air, metals, plants, and animals constitute A. matter B. stalactites C. stalagmites D. fusion E. None of these

Step 1: Convert the volume from $\text{cm}^3$ to $\text{dm}^3$. Given volume $V = 500 cm^3$. Since $1 dm^3 = 1000 cm^3$, $V = \frac{500 cm^3}{1000 cm^3/dm^3} = 0.5 dm^3$

Step 2: Use the formula for concentration to find the number of moles. The concentration ($C$) is given as $0.25 \text{ M}$ (which is $0.25 mol/dm^3$). The formula for concentration is $C = \frac{number of moles (n)}{Volume (V)}$. Rearranging for the number of moles: $n = C \times V$.

Step 3: Substitute the values and calculate the number of moles. $n = 0.25 mol/dm^3 \times 0.5 dm^3$ $n = 0.125 \text{ mol}$

Comparing this result with the given options, option B is $0.125$.

The final answer is $\boxed{B. 0.125}$.