The experiment results were surprising in fact they challenged long-held theories.
Correct: B
Option B correctly uses a semicolon to separate two independent clauses ('The experiment results were surprising' and 'they challenged long-held theories') that are linked by a conjunctive adverb ('in fact'). The conjunctive adverb itself should be followed by a comma. Option A creates a comma splice. Option C misplaces a comma. Option D uses a colon incorrectly; a colon usually introduces a list, explanation, or quotation, not typically used with a conjunctive adverb between two full clauses.