To solve this problem, we need to understand the concept of isostasy, which is the gravitational balance between the Earth's crust and mantle. When the thickness of the crust changes, the elevation also changes to maintain this balance.

The basic principle of isostasy can be illustrated by the model of Airy isostasy, where crustal blocks 'float' on the denser mantle beneath them. The elevation of these blocks can be calculated based on their thickness and density differences between the crust and mantle.

If the thickness of the crust is doubled, according to the principle of isostasy, the newly submerged part of the crust will displace an equal mass of the mantle below to maintain the balance. We must determine the elevation change in percentage when the crust's thickness is doubled.

Calculation:

• Let the original thickness of the crust be \(T\).
• On doubling, the thickness becomes \(2T\).
• The increase in thickness needs to adjust so that the mass remains balanced according to isostatic principles.
• The typical assumption is that around 50% of the newly added crust thickness will result in an increase in elevation because the rest will cause a corresponding deepening of the crust-mantle boundary.

Therefore, the elevation change is typically \(\frac{100}{5} = 20\%\), of the original elevated portion. This is because the crust 'floats' higher, just like an iceberg in water where a part of it is submerged and a part is above water.

Conclusion:

Among the given options (20%, 40%, 50%, 10%), the correct answer for the percentage change in elevation when the crust is doubled in thickness, adhering to isostatic balance, is 20%.