Answer: Ca
Step-by-step explanation:
From the periodic table, Ca is positioned below Mg.
Reason: Number of shells of Ca and Mg
Ca -> 4 shells
Mg -> 3 shells
Easier for Ca to lose two ions than Mg to form Ca2+ instead of Mg2+ due to weaker forces of attraction between the valence electrons and the positively charged nucleus. (Distance factor, the greater the distance between oppositely charged particles, the weaker the forces of attraction)
Hence, calcium is more reactive than Mg.
Can refer to the metal reactivity series under chapter Metals for a deeper understanding.