The answer is choice (d): the acceleration of the Voyager probe is 0 m/s^2.
According to the information given in this problem, there was no external force acting on the Voyager probe throughout the "tour." The engines were turned off, so there was no force provided by the engines, the outer space can be considered a vacuum, so there is no frictional force or resistance from the environment, and for the most part of the "tour," the Voyager probe was not close enough to a planet to experience a gravitational pull in one specific direction, so the average gravitational force on the probe was zero throughout the "tour."
The second Newton's Law states that in order for an object to experience acceleration, there has to be an external force acting on it:
`mveca = vecF`
Since the external force on the Voyager probe was zero (according to this problem), its acceleration, on average, was also zero.