Final answer:
Plato was bothered by the lack of mathematical order in the heavens as the irregular and complex movements of celestial bodies did not conform to the expected perfect circular paths. The movements of the Sun, Moon, and visible planets held particular significance, and ancient Greek and Renaissance astronomers struggled to model these motions within the framework of circles, eventually leading to more complex models like Ptolemy's geocentric system.
Step-by-step explanation:
What bothered Plato about the heavens was B. Lack of mathematical order. Ancient Greek astronomers, influenced by the ideas of Plato, sought to explain the motions of celestial bodies with geometric perfection. They were intent on finding order in the heavens, believing that the celestial motions had to be circular because the circle was considered a perfect shape. The complexity of actual planetary motion, which appeared to sometimes reverse or loop, was a source of frustration because it didn't conform to the expected perfect circular paths.
The movements of the seven objects in the sky (the Sun, the Moon, and five planets visible to the unaided eye) were especially significant and perplexing. Renaissance scientists, too, struggled with interpreting the observations made from Earth – and it became clear that to solve this complex problem, better observations and models were required. Ptolemy's model, which included dozens of circles moving around other circles, was an attempt to reconcile these observations with the belief in circular celestial motion. However, this model was complex and not reflective of the true simplicity of the celestial mechanics later described by Johannes Kepler's laws.