Stereochemistry

The correct order of stability for cyclohexane conformations is as follows:

• The chair conformation (A) is the most stable because it minimizes both steric and torsional strain.
• The twist boat conformation (C) is more stable than the boat conformation because it reduces some of the steric strain.
• The boat conformation (B) is less stable due to its "flagpole" interactions, which cause steric strain.
• The half-chair conformation (D) is the least stable, as it suffers from the highest steric and torsional strain.

Step 1: Analysis of Stereochemistry.
The structures (A) and (D) are non-mirror image stereoisomers that differ in the spatial arrangement of groups around a chiral center. This makes them diastereomers.

Step 2: Conclusion.
Thus, option (4) is correct, as (A) and (D) are diastereomers, while the other options do not represent accurate stereochemical relationships.

Final Answer: