Washing soda is commonly used to:

The correct answer is Remove grease and soften water.

Step-by-step Explanation:

1. Understanding Washing Soda: Washing soda is the common name for sodium carbonate (${\mathrm{Na}}_2{\mathrm{CO}}_3$). It is a chemical compound frequently used in domestic cleaning and laundry.

2. Water Softening: Hard water contains high concentrations of calcium (${\mathrm{Ca}}^{2+}$) and magnesium (${\mathrm{Mg}}^{2+}$) ions, which prevent soap from lathering effectively. Washing soda reacts with these dissolved calcium and magnesium ions to form insoluble precipitates (carbonate salts), thereby removing the ions from the water and softening it.

3. Removing Grease: Due to its alkaline nature, washing soda effectively binds to grease and oil molecules, breaking them down so they can be easily rinsed away with water.

4. Analyzing other options:
- Neutralise acids in human blood: This is incorrect; sodium bicarbonate (baking soda) or buffer systems in the body regulate blood pH, not washing soda, which is too harsh and toxic for ingestion.
- Increase acidity in baked foods: This is incorrect; baking soda (sodium bicarbonate) is used in baking as a leavening agent (basic, not acidic), whereas washing soda is not used in food production.
- Preserve meat for long periods: This is incorrect; common salt (sodium chloride) or sodium nitrate are typically used for preserving meats.