Which word reflects a sense of complacent foolishness?

The word that best reflects a sense of complacent foolishness is "fatuous." This term describes a person or behavior that is silly, foolish, or lacking in intelligence, often in a way that is accompanied by a sense of self-satisfaction or complacency. People who exhibit fatuous behavior might be blissfully unaware of their own ignorance or the gravity of a situation, embodying a kind of foolishness that is both naïve and unreflective.

In contrast, "turgid" refers to something that is swollen, inflated, or pompous, often lacking in clarity or substance rather than relating to ignorance. "Welter" describes a state of confusion or turmoil and does not imply a sense of foolish self-satisfaction. "Petrific" suggests something that is unable to change or is rigid, generally lacking the connotations of complacency or foolishness that "fatuous" carries. Thus, "fatuous" is the most fitting choice for the definition described.