Which of the following is known as alkaline earth metal?

Magnesium

The correct answer is option 3. Magnesium.

The element known as an alkaline earth metal among the options provided is Magnesium.

Alkaline earth metals are a group of elements in the periodic table that includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These elements share similar chemical properties, such as their ability to form divalent cations (ions with a +2 charge) and their tendency to react with water to form alkaline solutions. Magnesium falls into this category, making it the correct answer.