Which ingredient is known for its skin-softening properties and is a humectant derived from the decomposition of fats and oils?

The ingredient known for its skin-softening properties and classified as a humectant derived from the decomposition of fats and oils is indeed Glycerin. Glycerin is a versatile and widely used humectant in skincare products due to its ability to draw moisture from the environment into the skin, helping to maintain hydration and improve skin texture. It is a byproduct of the saponification process of fats and oils, which allows it to possess deeply moisturizing effects.

In addition to its moisturizing properties, Glycerin enhances the skin's barrier function and aids in preventing transepidermal water loss, promoting overall skin health. This makes it an essential ingredient in many cosmetic formulations aimed at achieving softer, more supple skin.

While other options present benefits for the skin, they do not fit the specific criteria of being derived from the decomposition of fats and oils or serving primarily as a humectant in the same way that Glycerin does.