The planet with the highest orbital period in our solar system is Neptune. However, since Neptune is not listed in your options, among the provided choices:
a. Mercury
b. Earth
c. Venus
d. Jupiter
The correct answer is neither of these options. All of them have shorter orbital periods than the gas giants like Saturn and Neptune. Of the four listed, Jupiter has the longest orbital period; it takes about 11.86 Earth years to complete one orbit around the Sun, which is longer than Mercury, Earth, and Venus.