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