**Explanation:**

A retarding admixture, also known as a water-reducing retarder or a retarder, is a type of admixture that is added to concrete to slow down the hydration process. It helps to keep the concrete workable for a longer period of time, allowing for easier placement and finishing.

**Tartaric acid, sugar, and gypsum** are commonly used as retarding admixtures in concrete. Here's a brief explanation of each:

1. **Tartaric acid:** Tartaric acid is a white crystalline organic acid that is commonly used as a retarder in concrete. It works by delaying the setting time of the concrete, which allows for better workability and finishing. It is particularly effective in hot weather conditions.

2. **Sugar:** Sugar is another common retarding admixture used in concrete. It acts as a retarder by slowing down the hydration process, allowing for extended workability. Sugar is inexpensive and readily available, making it a popular choice in construction.

3. **Gypsum:** Gypsum is a mineral compound that is commonly used as a retarder in concrete. It works by slowing down the hydration of the cement, which results in a longer setting time. Gypsum is often used in combination with other retarding admixtures to achieve the desired setting time and workability.

**Sodium silicate** is not a retarding admixture. It is actually an accelerating admixture, also known as a water glass. Sodium silicate is added to concrete to speed up the hydration process, reducing the setting time. It is commonly used in cold weather conditions or when faster strength development is required.

In conclusion, tartaric acid, sugar, and gypsum are retarding admixtures used to slow down the hydration process in concrete, while sodium silicate is an accelerating admixture used to speed up the process. Therefore, option D - sodium silicate - is not a retarding admixture.