Answer:
Li
Step-by-step explanation:
Is the element of the first group and is the second smallest of its group. Even Magnesium is in the same situation, but if you compare the atomic radius of lithium and magnesium, the first one is smaller.
Barium definitely is incorrect because is the one that has the biggest radius of its group.
Beryllium is the first of its group so it can not fit the description.