The answer is B: I think therefore I am.
Descartes is best known for this argument that he develops in his Discourse on the method. In this book, the French philosopher is concerned with reaching a proposition that would be indubitable and from which philosophy could aspire to build a form of knowledge that would be certain. As it goes, Descartes came to the conclusion that he could doubt everything except the fact that he was doubting. If such was the case, then one could be certain that, if one doubts, one is thinking, therefore, one must exist or be something (a thinking thing): I think, therefore I am.
