Final answer:
Mercury has the most elongated orbit of all the planets listed, with an eccentricity of 0.21, making its orbit the least circular.
Step-by-step explanation:
The planet whose orbit looks the least like a circle in our solar system is Mercury. Of the planets listed — Earth, Jupiter, Mars, and Mercury — Mercury has the most eccentric orbit with an eccentricity of 0.21. This means that Mercury's orbit is far more elongated compared to the nearly circular orbits of the other planets, which is why it looks the least like a circle. The eccentricity measures how much an orbit deviates from being a perfect circle, and since Mercury's eccentricity is the highest, its orbit is the most non-circular. Additionally, Mercury is known for being the smallest planet and has the shortest orbital period, taking only 88 days to complete one revolution around the Sun which also contributes to its high average orbital speed.