'I shall now therefore humbly propose my own thoughts, which I hope will not be liable to the least objection.'
1. B. Sarcastic
Swift understands the gamble he is taking by proposing this 'plan', so the heightened and formal language, 'hoping' that no one will object to it is sarcasm because he does not actually mean what he is saying.
2. C.The plan is meant to arouse objections and horror
Swift knows that the initial reaction to this piece will be one of outrage and horror, and that many people will object to the 'plan'. When he uses the phrase "I hope will not be liable to the least objection' it is clear irony because he knows how objectionable his plan actually is.