Answer: Magnesium (Mg)
Step-by-step explanation:
I attached a copy of a periodic table for reference.
When we look at the periodic table, go across to group 2 (Alkali Earth Metals) and then down to period 3, where we find Magnesium.
Interestingly, we could also gain this information from Magnesium's spdf electronic configuration: 1s²2s²2p⁶3s². With this, '3' tells us the period, and '²' tells the group.