No.3です。違いを知るには「同義語」の参考書を見るのがベストです。例えば私の持っている辞書では下記のように説明されています。
Synonyms: [foolish silly fatuous absurd preposterous ridiculous ludicrous]
These adjectives are applied to what is so devoid of wisdom or good sense as to be laughable. Foolish, the least emphatic and derogatory, usually implies poor judgment or lack of wisdom or soundness: a foolish young fellow; a foolish expenditure of time and energy. Silly suggests lack of point or purpose: a silly argument; silly mistakes; suggestions that aren't brilliant but aren't silly either. Fatuous applies especially to what is foolish in a vacuous, smug, and unconscious way: seems to take pride in making fatuous remarks. Absurd, preposterous, ridiculous, and ludicrous apply to what is risible because of a departure from reason, logic, or common sense: It would be absurd for us both to drive, since we're headed for the same destination. Preposterous describes what is contrary to reason or sense: “It would be preposterous to take so grave a step on the advice of an enemy” (J.A. Froude). Ridiculous refers to what inspires ridicule: “Clara's conceited assumption of a universal interest in her dull children was ridiculous” (Arnold Bennett). Ludicrous applies to what causes scornful laughter: It is ludicrous to call a simple split-level house a mansion.
「バカだね~」と日本人が冗談っぽく使うのに上記の単語を使うのは原則止めた方がよいと思います。個人攻撃的表現は英会話の語感では冗談にはなりにくい。むしろ
No way! No kidding! You don't mean it, do you? Come on! Bullshit!
など。真面目に忠告するなら "You're a fool." はあり得ます。
お礼
人に対してと、物に対してと、使い道が違うんですね★ そう言われてみれば、stupidは色々なシーンで見かけたりします。 sillyは最初の2つに比べると比較的やわらかいニュアンスなんですね。 ありがとうございます、参考にして人や物に使い分けたいと思います。