The most common nutritional deficiency worldwide is:

The question asks to identify the most common nutritional deficiency worldwide.

Common global nutritional deficiencies include iron deficiency, vitamin A deficiency, iodine deficiency, and zinc deficiency.

According to global health organizations such as the World Health Organization (WHO), iron deficiency is the most widespread nutritional deficiency globally, affecting a significant portion of the world's population, particularly women and children. It is the leading cause of anemia.

The most common nutritional deficiency worldwide is Iron deficiency.

$\text{Iron deficiency}$