To identify the unknown metal, we can calculate its density using the formula:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]
Given that the mass is 428 g and the volume is 20 cm³:
\[ \text{Density} = \frac{428 , \text{g}}{20 , \text{cm}^3} = 21.4 , \text{g/cm}^3 \]
Now let's compare this calculated density with the densities of the provided metals:
- Cobalt: 8.9 g/cm³
- Silver: 10.5 g/cm³
- Lead: 11.3 g/cm³
- Platinum: 21.4 g/cm³
- Chromium: 7.2 g/cm³
The calculated density of 21.4 g/cm³ matches the density of platinum.
Thus, the unknown metal is A. Platinum.