The full cell is used to represent a question mark indicating an omission in print. In this sentence, the question mark does not represent an omission. I would use the question mark (236) preceded by 456 -- punctuation indicator -- to show that the symbol is punctuation.
I was challenged on my advice to use 456 before the question mark. Rule VI§38 says that a punctuation indicator is not used at the beginning of a braille line or after a space. In those circumstances the mode of punctuation is considered to be literary. Thus, if we follow the literary rules, BF Rule 6§1b says a dot 4 should be inserted before a freestanding punctuation mark. I apologize for my error.