Copernicus assumed that planetary orbits were circles in his heliocentric model. He regarded uniform circular motion as the ideal form of celestial movement, an assumption inherited from ancient Greek philosophy and long embedded in European astronomy.
His model moved the Sun near the center and made Earth one of the orbiting planets, but it did not yet describe the Solar System with modern accuracy. To match observations—especially planets' changing speeds and apparent retrograde motion—Copernicus combined circular paths with smaller circles called epicycles. His system could reproduce planetary positions, although it remained mathematically complicated.
The circular-orbit assumption was eventually shown to be incorrect. Using observations collected by Tycho Brahe, Johannes Kepler discovered in the early 17th century that planets travel in ellipses, with the Sun at one focus. Kepler's laws also showed that planets move at varying speeds rather than uniformly. Isaac Newton later explained these elliptical orbits through universal gravitation.
Thus, circles accurately describe what Copernicus proposed, even though modern astronomy uses elliptical orbits. The ellipse option refers mainly to Kepler's later refinement, not Copernicus's original model.