Answer:
Strontium
Step-by-step explanation:
Elements are classified into groups and periods. Elements that are grouped together in the same group are said to have the same number of valence electrons, hence, they behave the same way chemically. This is the case of Calcium which reacts with Chlorine to form CaCl2. Calcium belongs to Group II in the periodic table, hence, possesses two valence electrons which it transfers to chlorine atoms.
Strontium (Sr) will behave the same way like Calcium because they belong to the same group in the periodic table, meaning that they both have two valence electrons, which they'll transfer. Strontium will form compound SrCl2 with Chlorine.