Question

which excerpt from \the enigma machine\ supports the idea that allied forces expected the germans to complicate their coding system?
the enigma machine was invented at the end of world war i by german engineer arthur scherbius. he used rotors and an electrical pathway in his design.
the telegram was intercepted and deciphered by british intelligence, and it caused such outrage in the us that it helped generate public support for the united states entry into the war.
after 1938, when the germans introduced additional rotors and plugs to increase the number of possible combinations, polish and british cryptologists had already developed alternative methods of recovering the daily keys.
the two main weaknesses of the enigma machine were careless operators and the distribution of the daily key. the daily key was printed on paper, and it had the potential of being captured en route to the field.

Explanation:

The key idea is that Allied forces (Polish and British cryptologists here) had prepared (developed alternative methods) because they expected the Germans to make the coding system more complex (introduce additional rotors and plugs). The other options either talk about invention (first option), an event leading to US entry (second option), or weaknesses (fourth option) which are not related to expecting complication of the coding system.

Answer:

After 1938, when the Germans introduced additional rotors and plugs to increase the number of possible combinations, Polish and British cryptologists had already developed alternative methods of recovering the daily keys.