Step-by-step Solution:

Quick Solution

This puzzle can be solved with a quick, direct deduction using the specific conditions given in the second part of the question.

1. Use the Main Clues to Find A's Position

• The question provides a specific condition to start with: "C is standing on Step 6".
  
• We also have a general rule from the initial description: "The step number, on which A is standing, is two less than that of C". This can be written as the equation: A = C - 2.
  

By substituting C's definite position (6) into the rule, we can directly find A's position:

A = 6 - 2
A = 4

This immediately tells us that A must be standing on Step 4.

2. Verification (Optional)

We can quickly check if this is consistent with the other rules to confirm our answer:

• The clue "two steps in between A and D" means their step numbers are 3 apart. If A is on 4, D must be on step 1.
• The clue "B is one more than D" means if D is on 1, B must be on step 2.
• The final positions are C=6, A=4, B=2, D=1. This is a valid arrangement with no conflicts.

Final Answer: A must be standing on step 4.