Irony is a figure of speech or literary device in which what actually happens is completely different or opposite from the reader or audience's expectations.
Option A is correct. We know that snow melts under the heat, and Summer is supposed to be the hottest season. So it is indeed ironic that a snowman (who should consider Summer his enemy) actually loves the summer season. Thus, there is irony in this situation.
Option B is incorrect. The sentence is a pun. A pun is usually a joke that makes a play on words. It uses words that have several meanings or words that sound similar but have different meanings. In this sentence, the speaker makes a play on the word "struck" and its association with lightning. We know that lightning strikes, "struck" also means a sudden realisation . This is the pun.
Option C is incorrect as it a paradox. A paradox is different from irony. A paradox is a statement that may seem absurd or contradictory but yet can be true, or at least make sense. We can see this happening in option C. The speaker says that he knows that he knows nothing. However, the fact that he says that he knows this means that he actually knows this one thing . Which goes against the fact that he knows nothing .
Option D is incorrect as the sentence contains an oxymoron, not irony. An oxymoron is a figure of speech that combines two words which contradict each other, but still makes sense. For example, "dumb genius". The oxymoron contained in option D is "seriously funny" where the meaning of "serious" is the complete opposite of "funny".