Choice C; the period is unchanged.
Work Step by Step
We see from equation 13.12 that the period depends only on the mass of the central body (i.e, the mass of the star), not on the planet’s mass. In other words, a planet that is 3 times more massive feels a force that is three times greater, but is also three times harder to accelerate. The planet's acceleration, and therefore its motion, ends up being the same.