Answer
If the semi-major axis $a$ is increased four-fold, then $T$ is increased by eight-fold.
Work Step by Step
Kepler's third law states that ${T^2}$ is proportional to ${a^3}$. It means that $T$ is proportional to ${a^{3/2}}$. If the semimajor axis $a$ is increased four-fold, then $T$ is increased by eight-fold.
