The correct option is C Postscript
'Postscipt', Option C is 'anything written in a letter after it is signed'. Option A, Posterity means all the future generations of people. Option B 'Postdiction' involves explanation after the fact. Option D, 'Corrigendum' is a mistake in printed text that needs to be corrected. Thus, the correct word of the phrase is option C, Postscript and other options A, B and D are incorrect.