**Core Concept**
Non-heme iron and heme iron are two forms of dietary iron that have different bioavailability. Non-heme iron is found in plant-based foods such as beans, lentils, and fortified cereals, while heme iron is found in animal-based foods such as meat, poultry, and fish. The bioavailability of non-heme iron is generally lower than that of heme iron due to various factors.

**Why the Correct Answer is Right**
The low bioavailability of non-heme iron is primarily due to its poor solubility in the acidic environment of the stomach and its limited absorption in the small intestine. Non-heme iron is also more susceptible to binding with dietary phytates, oxalates, and polyphenols, which can inhibit its absorption. In contrast, heme iron is more readily absorbed due to its higher solubility and its ability to bypass the acidic environment of the stomach. Heme iron is also less likely to bind with dietary inhibitors, allowing for more efficient absorption.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide a specific reason for the low bioavailability of non-heme iron.
* **Option B:** This option is incorrect because it suggests that the low bioavailability of non-heme iron is due to its high solubility, which is actually the opposite of the correct answer.
* **Option C:** This option is incorrect because it implies that the low bioavailability of non-heme iron is due to its high reactivity, which is not a relevant factor in this context.

**Clinical Pearl / High-Yield Fact**
To enhance the bioavailability of non-heme iron, it is recommended to consume vitamin C-rich foods along with iron-rich plant-based foods. Vitamin C can help to reduce the binding of non-heme iron with dietary inhibitors, allowing for more efficient absorption.

**Correct Answer:** D.