What is the central idea of this paragraph? the code book

What is the central idea of this paragraph? Breaking RSA code is the greatest challenge for cryptanalysts. One of the author's purposes in The Code Book is to explain different types of codebreaking to his readers.

the correct answer is option d. the nature of democracy. on november 19, 1863, president abraham lincoln delivered his famous gettysburg address in gettysburg, pennsylvania. in his speech, rather than focusing on details of the war (soldiers, casualties, aftermath of battle), lincoln invoked the very nature of democracy, that power ultimately comes from the people, when stating, “government of the people, by the people, for the people shall not perish”. these words mean that democracy is a government by the people (citizens) in which the supreme power is vested in the people and exercised directly by the people or by their elected representatives for the benefit of all citizens. the survival of democracy in the united states is one of the reasons why this speech so significant today.