💡 Detailed Explanation
The adverb 'つい' (tsui) means 'unintentionally', 'carelessly', or 'accidentally' (often doing something you know you shouldn't because of weak willpower). The correct and natural sentence is 'ダイエット中なのに、ついケーキを食べてしまった' (Even though I am on a diet, I ended up eating cake by impulse/accidentally). The other choices represent grammatically incorrect and unnatural uses of 'つい'.