「」。
He wrote 'Have a hot dog' underneath the cartoon.
Literal
Cartoon's under [at-に] 「hot dog [object-を] please-have」 [quotative-と] wrote [explanatory-のである].
Two things stand out. The quoted material 「ホット・ドッグをどうぞ」 uses Japanese quote brackets 「」 around a fragment of direct speech. And the ending のである is the formal written explanatory — the である-variant of ~のだ — giving the sentence a deliberative, expository tone ('this is what happened, you see').