Final answer:
The element found in the minerals celestite and strontianite is strontium, which is part of the alkaline earth metals group in the periodic table.
Step-by-step explanation:
The elements found in the minerals celestite and strontianite are indeed part of the alkaline earth metals group, also known as Group 2 of the periodic table. This group includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Specifically, the mineral celestite is comprised of strontium sulfate (SrSO4) and strontianite consists of strontium carbonate (SrCO3). Therefore, the correct answer to the question is B. Strontium.