The correct answer is Ba (Barium) because it has the smallest ionization energy due to its position in the periodic table. As you move down a group, the ionization energy decreases because the outermost electrons are further from the nucleus and are thus less tightly held, making them easier to remove.

In contrast, Li (Lithium) has a high ionization energy as it is at the top of Group 1; Al (Aluminum) has a moderate ionization energy, higher than barium; and Ca (Calcium) has a lower ionization energy than aluminum but still higher than that of barium due to being higher in the group. Therefore, the trend of decreasing ionization energy down the group clearly shows that barium has the lowest value among the options presented.

Lithium has a relatively high ionization energy compared to heavier elements in the same group, as it is located at the top of Group 1.

Aluminum has a moderate ionization energy, higher than that of alkaline earth metals and heavier elements in its group.

Calcium has lower ionization energy than Al, but it is still higher than that of barium because it is positioned higher in Group 2.

Barium has the smallest ionization energy among the given options because it is located lower in Group 2, where ionization energy decreases.