What aspect of God's existence does Aquinas challenge regarding Anselm’s argument?

Aquinas challenges the notion that God's existence is self-evident as proposed by Anselm's argument. Anselm posits that God’s existence can be understood through the concept of a being greater than which cannot be conceived; he holds that this being must exist in reality because existing in reality is greater than existing merely in the understanding.

Aquinas, however, contends that the existence of God is not self-evident and cannot be simply understood by reason alone without additional proof. He emphasizes the need for empirical evidence and philosophical arguments to demonstrate God's existence, such as the Five Ways, which provide a rational basis for belief rather than requiring acceptance of God's existence based solely on conceptual reasoning.

This highlights a fundamental difference in their approaches to theology and philosophy; Anselm’s argument leans heavily on a priori reasoning, while Aquinas adopts an a posteriori stance that relies on observation and experience to substantiate faith. The other choices do not capture Aquinas's specific challenge as well as this option does.