**Core Concept**
Iodine fortification of salt is a public health strategy to prevent iodine deficiency disorders (IDD), which can lead to conditions such as goiter, hypothyroidism, and cretinism. The World Health Organization (WHO) recommends a minimum of 15-20 mg of iodine per kilogram of salt to achieve optimal iodine intake.

**Why the Correct Answer is Right**
The recommended iodine content in salt is crucial for maintaining adequate thyroid hormone production. Iodine is essential for the synthesis of thyroxine (T4) and triiodothyronine (T3) in the thyroid gland. The optimal iodine intake helps to prevent iodine deficiency, which can lead to decreased thyroid hormone production and subsequent hypothyroidism. The recommended iodine content in salt is designed to provide sufficient iodine to the population, especially in areas where iodine-poor soil and water are prevalent.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is below the recommended minimum iodine content. Iodine deficiency can lead to severe health consequences, including goiter, hypothyroidism, and cretinism.
**Option B:** This option is incorrect because it exceeds the recommended maximum iodine content. High iodine intake can lead to iodine-induced hyperthyroidism, which can cause symptoms such as tremors, palpitations, and weight loss.
**Option C:** This option is incorrect because it does not specify the correct unit of measurement (mg/kg). Iodine content in salt should be expressed in milligrams per kilogram to ensure accurate measurement.

**Clinical Pearl / High-Yield Fact**
Iodine fortification of salt is a cost-effective and sustainable public health strategy to prevent IDD. It is essential to maintain adequate iodine intake, especially in areas where iodine-poor soil and water are prevalent. The recommended iodine content in salt should be clearly labeled on the packaging to ensure that consumers are aware of the iodine content.

**Correct Answer:** D. 15-20 mg/kg