Answer:
If only one of these statements is true, #17 robbed the bank.
Step-by-step explanation:
1st Scenario: #26 tells the truth
If #26 is telling the truth, that means #14 is guilty and, consequently, lying. However, that would also mean that #17 is telling the truth and then more than one statement would be true.
2nd Scenario: #17 tells the truth
If #17 is telling the truth, #17 is innocent, but then again either #26 or #14 are lying and two statements would be true.
3rd Scenario: #14 tells the truth
If #14 is telling the truth, #14 is innocent and, consequently, #26 is lying. That leaves us with #17 claiming innocence, but since only #14 can be telling the truth, #17 is lying and robbed the bank.