Final answer:
The sentence that is punctuated incorrectly as to its introductory element is (d) Our best guesses proving incorrect, we left the safe for someone else to crack.
Step-by-step explanation:
The sentence that is punctuated incorrectly as to its introductory element is:
d) Our best guesses proving incorrect, we left the safe for someone else to crack.
In this sentence, the introductory element is "Our best guesses proving incorrect." However, there should be a comma after this introductory phrase to separate it from the main clause "we left the safe for someone else to crack." The correct punctuation should be:
Our best guesses proving incorrect, we left the safe for someone else to crack.