**Core Concept**
The Recommended Dietary Allowance (RDA) of iodine refers to the average daily intake sufficient to meet the nutritional needs of most healthy individuals, particularly for thyroid hormone synthesis and function. Iodine is a critical micronutrient for the production of thyroid hormones, and deficiency can lead to goiter, hypothyroidism, and developmental delays in fetuses and children.

**Why the Correct Answer is Right**
The RDA for iodine in adults is **150 micrograms per day** as recommended by the National Institutes of Health (NIH) and the World Health Organization (WHO). This amount supports normal thyroid function and prevents iodine deficiency disorders. The value is based on population-level requirements, accounting for physiological needs across age groups and life stages.

**Why Each Wrong Option is Incorrect**
Option A: 300 microgram – This is higher than the RDA and may be used in specific clinical conditions like thyroiditis, but it is not the general population RDA.
Option B: 500 microgram – This is significantly above the RDA and is not recommended for routine use; it may lead to hyperthyroidism or thyroid dysfunction.
Option D: 50 microgram – This is below the RDA and insufficient to meet physiological needs, especially in pregnant or lactating women.

**Clinical Pearl / High-Yield Fact**
Iodine deficiency remains a global public health issue, especially in mountainous or inland regions. Pregnant women require **220 micrograms/day** due to increased demand, and RDA is higher during pregnancy and lactation. Always consider iodine supplementation in endemic areas or for high-risk groups.

✓ Correct Answer: C. 150 microgram