Equivalent outer positions in glycerol limit distinct arrangements

Steps:

• Glycerol has three attachment sites: two equivalent primary (outer) positions and one distinct secondary (central) position.
• Choose fatty acid for central position: 3 options (L, O, or P).
• Assign remaining two fatty acids to outer positions: 1 way, as swapping them yields identical molecules due to equivalence.
• Total distinct arrangements: 3.

Why A is correct:

• The two primary positions on glycerol are equivalent (prochiral), so only regioisomers based on central chain position are distinct, giving 3 unique structures.

Why the others are wrong:

• B: Overcounts by treating outer positions as fully distinct without equivalence.
• C: Incorrect; no basis for 5 permutations with three unique chains.
• D: Wrong; 3! = 6 full permutations, but halved to 3 by equivalence, not 9 (perhaps erroneous 3×3).

Final answer: A