Which phrase accurately replaces the missing part: I don't know why Fred made such an extraordinary decision?

The phrase "What prompted Fred to" is the most appropriate choice for replacing the missing part in the sentence. It maintains the original context, emphasizing the causative aspect of Fred's decision. "Prompted" suggests an external influence or reason behind Fred's extraordinary decision, aligning well with the inquiry about why he made such a choice.

In contrast, "What inspired Fred to" suggests a positive motivation, which might not necessarily capture the unexpected nature of an "extraordinary decision." "What shocked Fred to" is grammatically incorrect and does not fit well within the context—people are not shocked into making decisions. Lastly, "What deterred Fred from" implies that something prevented him from making a decision, which is the opposite of what the sentence suggests. Therefore, the phrase that best captures the inquiry into his extraordinary decision is "What prompted Fred to."