Which point defect in crystals of a solid decreases the density of the solid?

Explanation

To determine which point defect in crystals of a solid decreases the density of the solid, we can follow these steps:

### Step-by-Step Solution:

1. **Identify Point Defects**: Understand that point defects in crystals include vacancies, interstitials, and substitutional defects.

2. **Focus on Density**: Recall that density is defined as mass per unit volume (Density = Mass/Volume). To decrease the density of a solid, we need to either decrease the mass while keeping the volume constant or increase the volume while keeping the mass constant.

...