Step-by-step Solution:

Quick Solution

To solve this puzzle, we use the specific condition given in the question ("D is standing on step 1") and then apply the general rules to find all possible and valid positions for A.

1. Place D and B

• The question gives us a starting point: D is on Step 1.
• We then use the general rule: "The step number on which B is standing is one more than that of D" (which means B = D + 1).
• Therefore, B's step is 1 + 1 = 2. So, B is on Step 2.

With steps 1 and 2 occupied, the remaining available steps for the other people are {3, 4, 5, 6}.

2. Find Possible Positions for A and C

- The other general rule is "The step number, on which A is standing, is two less than that of C" (which can be written as C = A + 2).
- This means A and C must occupy two steps that are separated by exactly one other step (e.g., if A is on 3, C must be on 5).
- We need to check the available steps {3, 4, 5, 6} to see which placements are possible for A.

• Possibility 1: Could A be on Step 3? If so, C would be on Step (3 + 2) = 5. Since both steps 3 and 5 are available, this is a valid possibility.
• Possibility 2: Could A be on Step 4? If so, C would be on Step (4 + 2) = 6. Since both steps 4 and 6 are available, this is also a valid possibility.
• (If A were on Step 5, C would need to be on Step 7, which doesn't exist, so no further positions are possible for A).

3. Conclusion

Based on the rules and the given condition, there are two possible steps where A could be standing.

Final Answer: A could be standing on 3 or 4 only.