The correct answer is B. I had sixty-seven dollars until someone—we’re still not sure who—stole my wallet from my back pocket.
Explanation:
In grammar, there are different punctuation marks including the period (.), comma (,), hyphen (-) and the dash(—) each one used in different cases and with different purposes and grammatical rules. In the case of the dash or emdash (—), which the punctuation mark presented in the sentences of the question, this punctuation mark is used to replace other punctuation marks such as commas or parentheses mainly to introduce words of clauses that work as extra information but are not essential to the main sentence with a stronger break than in the case of parentheses or commas.
Considering this, in the case of the sentence presented the information that can work as a clause for extra information is "we’re still not sure who" and therefore the sentence that uses correct punctuation is " I had sixty-seven dollars until someone—we’re still not sure who—stole my wallet from my back pocket" as in this sentence the em dashes replace the use of commas to add further information that does not directly belong to the main sentence " I had sixty-seven dollars until someone stole my wallet from my back pocket".
