What is the primary function of selenium in the human body?

Selenium is primarily recognized for its role as an antioxidant in the human body. It is an essential trace mineral that helps neutralize free radicals, which are unstable molecules that can cause cellular damage. By doing so, selenium plays a crucial role in protecting cells from the oxidative stress that can lead to various chronic diseases, including heart disease and certain cancers.

Additionally, selenium is important for the proper functioning of the immune system and contributes to thyroid hormone metabolism. The fact that it acts in conjunction with other antioxidant nutrients, such as vitamin E, further underscores its significance in maintaining overall health and preventing oxidative damage.

In contrast, the other options focus on roles that are not primarily attributed to selenium. While calcium absorption is relevant to minerals like vitamin D, muscle growth is more closely associated with nutrients like protein and certain vitamins, particularly the B vitamins. Regulation of blood sugar levels is primarily the function of insulin and other hormones, rather than selenium itself. Thus, option B best encapsulates the key function of selenium in human health.