I missed this answer when drilling the passage and my explanations are included below. If I am missing anything or wrong about something I would appreciate good criticism. It took me a few minutes before I could find the reason that E was correct.
A: Unsupported. Lines 40-44 describe that the government is only held responsible for compensating the other party if they modify the terms of the contract. This answer choice does not clearly imply that the contract has been modified in any way.
B: Unsupported, it says that an administrative contract may include certain terms not found in private contracts (line 38). However you cannot infer that this means that the contractor gains guarantees that are not provided normally in private civil contracts since no information is given that proves that one way or another.
C: Being ratified is one of the two different ways mentioned that it can be designated as an administrative contract. As follows, it is not required.
D: Perhaps the critics/advocates mentioned around lines 19-27 would agree with this. This seems to run counter to what the author would think however.
E: Best answer choice of the group. When the administrative contract is designated by statue it has to follow certain applicable rules upon entering into an agreement with the government. Can be supported from lines 31-35. In the second method it is explicitly mentioned in lines 36-39.