In the given series elements of the series have been arranged in the following manner: First term = 1 Second term = 1 × 2 = 12 Third term = 1 × 2 × 3 = 123 Fourth term = 1 × 2 × 3 × 4 = 1234 Therefore, fifth term in place of the question mark = 1 × 2 × 3 × 4 × 5 = 12345