'Known secret' is an example of -

Known secret is an example of an oxymoron.

An oxymoron is a figure of speech in which two words with opposite or contradictory meanings are placed together. The phrase “known secret” combines known and secret, which clash in meaning, so it is an oxymoron.

Why the other options are incorrect:

  
• Open secret refers to something that is widely known, even if officially treated as secret. It is not the correct literary term here.
  
• Romanticism is a literary and artistic movement, not a figure of speech.
  
• Nazism is a political ideology, unrelated to the meaning of the phrase.

So, the correct answer is oxymoron.