**Core Concept**
The question is testing the concept of mucous membrane permeability and the absorption of various substances. Mucous membranes are semi-permeable barriers that allow the passage of certain molecules while restricting others. The absorption of substances across mucous membranes depends on their molecular size, lipid solubility, and charge.

**Why the Correct Answer is Right**
The least absorbed substances from mucous membranes are those with large molecular sizes, high charge, or low lipid solubility. These include ions, large proteins, and polar molecules. The mucous membrane's selective permeability is mainly due to the presence of tight junctions, which restrict the passage of large molecules. Additionally, the mucous membrane's epithelial cells have specific transport mechanisms that facilitate the absorption of certain substances while preventing the passage of others.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is a common misconception that all substances are absorbed equally from mucous membranes. In reality, the absorption of substances depends on their specific properties.
* **Option B:** This option is incorrect because it is a general statement and does not specify the type of substance that is least absorbed. The correct answer should be a specific type of substance that is known to have poor absorption from mucous membranes.
* **Option C:** This option is incorrect because it is a type of substance that is actually well-absorbed from mucous membranes. Substances with high lipid solubility, such as steroids, are readily absorbed from mucous membranes.

**Clinical Pearl / High-Yield Fact**
The mucous membrane's selective permeability is a critical factor in the development of various diseases, including inflammatory bowel disease and mucosal infections. Understanding the mechanisms of mucosal absorption is essential for the development of new treatments and therapies for these conditions.

**Correct Answer: D.**