Final answer:
Radium (Ra) is a representative element as it is an alkaline earth metal in the s-block of the periodic table, while Actinium (Ac) is not a representative element since it's part of the d-block.
Step-by-step explanation:
The question asks which element is a representative element among Ra (radium) and Ac (actinium). Representative elements are those found in the s or p blocks of the periodic table. Since Ra is an alkaline earth metal, it belongs to the s-block, specifically group 2. This makes it one of the representative elements. In contrast, Ac is the first member of the fourth transition series in the d-block, which means it is not considered a representative element. Therefore, the correct answer is Ra (radium).