Final answer:
Descartes believed that knowledge is option (c). certain, stemming from clear and distinct ideas that are indubitable, as part of a rationalist perspective.
Step-by-step explanation:
According to Descartes, knowledge is certain. Descartes' philosophy posits that knowledge comes from clear and distinct ideas, which are not capable of being doubted. These ideas form the foundation of all knowledge, which is attained by acts of reasoning. In essence, if a subject can conceive something clearly and distinctly in their minds, they can assume it to be true, as God would not deceive them.
This approach underlies Descartes' rationalist view that we have innate knowledge of things such as the self, God, and certain aspects of the world. Therefore, among the given options, the one that corresponds with Descartes' position is 'certain'.