**Core Concept**
Par boiling of paddy is a process that involves partial cooking of paddy in water under pressure to inactivate enzymes that cause spoilage and improve its storability. This process affects the physical and chemical properties of paddy, making it more suitable for storage and processing.

**Why the Correct Answer is Right**
The correct answer is related to the inactivation of enzymes, specifically alpha-amylase and lipase, which are responsible for the breakdown of starch and fats in paddy. Par boiling denatures these enzymes, preventing them from causing spoilage and improving the shelf life of paddy. This process also helps in increasing the moisture content of paddy, making it easier to mill and process.

**Why Each Wrong Option is Incorrect**
**Option A:** **Nutrient retention** - While par boiling does help in retaining some nutrients, it is not the primary benefit of this process.
**Option B:** **Pest control** - Par boiling does not directly affect pest control, which is more related to storage conditions and handling practices.
**Option C:** **Grain yield** - Par boiling does not directly impact grain yield, which is more related to factors like soil quality, irrigation, and crop management.

**Clinical Pearl / High-Yield Fact**
Par boiling of paddy is an example of a pre-processing technique that can improve the quality and storability of food grains. This process is particularly useful for crops that are prone to spoilage, like wheat and rice.

**Correct Answer:** A. Nutrient retention.