**Core Concept**
The underlying principle being tested is the physiological adaptation after surgical resection of the ileum, specifically in the context of **vitamin B12 absorption** and **bile salt recycling**. The ileum plays a crucial role in the absorption of these essential nutrients.

**Why the Correct Answer is Right**
After ileal resection, the primary physiological change is the impairment of **vitamin B12** and **bile salt** absorption, as the ileum is the primary site for their absorption. This is because **intrinsic factor**, a protein necessary for vitamin B12 absorption, binds to vitamin B12 in the ileum, and **bile salts** are recycled in the ileum to maintain the **enterohepatic circulation**.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not directly relate to the primary function of the ileum.
**Option B:** Similarly, this option does not accurately describe the primary physiological change after ileal resection.
**Option C:** This option is also incorrect as it does not specifically address the role of the ileum in nutrient absorption.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that patients who have undergone ileal resection are at risk of developing **vitamin B12 deficiency**, which can lead to **megaloblastic anemia** and **neurological symptoms**.

**Correct Answer:** D. Malabsorption of vitamin B12.