Final answer:
The escape velocity from a planet with twice the radius and the same mean density as Earth would be approximately 15.5 km/s, as the calculation is based on the square root of the mass-radius ratio.
Step-by-step explanation:
The escape velocity from a planet is determined by the formula ve = √(2GM/R), where G is the gravitational constant, M is the mass of the planet, and R is the radius of the planet. Since the escape velocity from Earth is about 11 km/s and the question states that the new planet has the same mean density as Earth but twice the radius, we can calculate the escape velocity for this new planet.
The mass of the planet, given constant density, will be increased by a factor of 8 (since volume scales with the cube of the radius), and the radius is doubled, resulting in an escape velocity that is √2 times that of Earth's. Since √2 is approximately 1.41, the escape velocity from the new planet will be 11 km/s × 1.41, which is approximately 15.5 km/s. Therefore, the correct answer is D. 15.5 km/s.