Hint: The Solar System is the gravitationally bound arrangement of the Sun and the items that circle it. Of the objects that circle the Sun, the most prominent are the eight planets, and the rest being smaller bodies like dwarf planets.
Option A: Discovered in 1930, Pluto was labeled as the ninth planet of the Solar System. However, in 2006, The International Astronomical Union (IAU) declared that Pluto is no longer a planet since it did not fulfill the three criteria the IAU uses to define a full-sized planet. It is 1188.3 km in radius. Therefore Pluto is called a dwarf planet now.
Option B: Mercury is the closest planet to the Sun in the Solar System, and naturally has the shortest orbital period among all the planets. The radius of Mercury is 2439.7 km, making it the smallest planet.
Option C: Venus is the second planet from the Sun, and comes right before Earth. The radius of Venus is 6051.8 km.
Option D: Jupiter is the largest planet of the solar system. It is fifth in line from the Sun. Jupiter is best known for its stripes, Great Red Spot, and its 79 known moons. The radius of Jupiter is 69,911 km.
Hence the correct answer is (B) Mercury.
Note: It must be noted that Pluto was considered the smallest planet till 2006 because it was categorized as a planet back then. It is no longer considered a full-sized planet. Therefore, as of 2020 the smallest planet in the Solar System is Mercury and you must not get confused between the two.