Step 1: Picture what compaction physically does.
Compaction applies mechanical energy, rolling, tamping or vibration, that rearranges soil particles into a denser packing by driving out the air trapped between them.
Step 2: Track each option through this process.
Strength and stability are both improved, not reduced, once particles interlock more tightly, so those two options move in the wrong direction. Dry density rises as more solid mass gets packed into the same volume, so that option also moves the wrong way.
Step 3: Identify what actually drops.
Void ratio is defined as the volume of voids divided by the volume of solids. As air is expelled and particles pack closer together, the void volume shrinks while the solid volume stays fixed, so the void ratio necessarily decreases. This inverse link between compaction and void ratio is exactly why compaction test curves are also used as a proxy for tracking dry density.
\[ \boxed{\text{Void ratio}} \]