The correct answer is C: they're.
So the complete sentence would be: "I love to go to Sweet Treats for dessert because they're milkshakes are irresistible." However, a more grammatically correct version of the sentence would be: "I love to go to Sweet Treats for dessert because their milkshakes are irresistible."
If you want to stick strictly with the options provided, C is the best fit in terms of spelling and context relevance, but it still includes an error. Option D would be the most grammatically correct version if that were changed to the last part.