Fist and seafood are known to be very rich in magnesium. Magnesium is an essential mineral that plays a crucial role in various bodily functions. It is primarily involved in energy production, nerve function, muscle contraction, and the synthesis of DNA and proteins.

Magnesium is especially important for maintaining a healthy cardiovascular system, as it helps regulate blood pressure and cholesterol levels. It also supports the functioning of the immune system, promotes bone health, and aids in the absorption of other minerals like calcium and potassium.

While options like iron, calcium, and sodium are also found in some fish and seafood, magnesium is particularly abundant in these types of food. Iron is more commonly associated with red meat and leafy greens, while calcium is often found in dairy products and certain fruits and vegetables. Sodium, on the other hand, is primarily linked to processed foods and added salt.

Therefore, the correct answer is option 1 - magnesium.