Answer: cant
Explanation: In this line, the speaker expresses assurance regarding the individual's involvement in the heist. The modal verb "must" expresses a strong deduction or conclusion based on facts or logical reasoning. To construct a negative deduction or conclusion, we employ the contraction "can't" (cannot) followed by the base form of the verb "have been involved." So the phrase should be: "He can't have been involved in the robbery."
