I believe the correct answer is sentences 2,5 and 6.
Complex sentences are composed of one independent clause and at least one dependent clause. The examples from "The Enigma Machine” which are complex sentences are:
2. For example, Bletchley Park was crucial in the Battle of the Atlantic, which lasted six years.
5. This was provided in a codebook, which was a monthly list of daily keys distributed to various networks in the German military.
6. Since the Enigma machine could encrypt text into over 150 trillion possible combinations, Germans were convinced the codes were not decipherable.