Considering LiBH₄ reduces an ester group to the corresponding alcohol and does not reduce a carboxylic acid group, the correct statement about the major products P, Q, R and S is

Correct Answer: P & Q are identical, and R & S are diastereomers.

To determine the stereochemical relationship between the major products P, Q, R, and S, we analyze the chemoselectivity of the reducing agent Lithium Borohydride (LiBH₄) along with the symmetry and configuration of the resulting compounds.

Step 1: Chemoselectivity of LiBH₄
Lithium borohydride (LiBH₄) is a selective reducing agent. As given:
1. It selectively reduces an ester group (-COOR) to a primary alcohol (-CH₂OH).
2. It does not reduce a carboxylic acid group (-COOH).

Step 2: Analysis of Products P and Q
When cyclic or acyclic stereoisomers containing both an ester group and a carboxylic acid group undergo reduction by LiBH₄, only the ester group undergoes conversion to -CH₂OH, while the -COOH group remains unchanged.

For the pair of reactions forming products P and Q:
• In P and Q, the reduction yields compounds where one substituent is -COOH and the other converted substituent is -CH₂OH.
• Because the resulting functional groups at the two stereocenters/positions are identical (-CH₂OH and -COOH swapped depending on orientation/rotation), rotating or flipping the molecule reveals a plane of symmetry or superimposability.
• Thus, P and Q represent the same chemical entity in different spatial orientations, making P & Q identical.

Step 3: Analysis of Products R and S
For the pair of products R and S:
• The relative stereocenters (cis vs. trans orientations of the substituents) remain fixed during the reaction because the reduction of the ester group occurs without inversion or retention affecting the stereogenic carbon backbone.
• One product pair retains a cis-relationship between substituents, while the other retains a trans-relationship.
• Non-superimposable stereoisomers that are not mirror images of each other are classified as diastereomers.
• Therefore, R & S are diastereomers.

Conclusion:
Combining both findings, P & Q are identical, and R & S are diastereomers.