1. **State the problem:** We have the sequence 1, 12, 23, 34, 45, ? and need to find the number that fits in place of the question mark. 2. **Analyze the pattern:** Look at the differences between consecutive terms: $$12 - 1 = 11$$ $$23 - 12 = 11$$ $$34 - 23 = 11$$ $$45 - 34 = 11$$ 3. **Identify the rule:** Each term increases by 11 from the previous term. 4. **Find the missing term:** Add 11 to the last known term: $$45 + 11 = 56$$ 5. **Conclusion:** The missing number is 56, which corresponds to option d. Final answer: **56**