Explanation:

- Let’s break it down. Mendeleev called the unknown element below aluminium “eka-aluminium” because no one had discovered it yet. He predicted its properties pretty well, based on his periodic table.

- Option 1: Boron — Not the one; boron sits way over on the left of aluminium in the periodic table and isn’t similar in properties.

- Option 2: Geranium — Small typo here: the correct element is "germanium," but germanium replaced eka-silicon, not eka-aluminium.

- Option 3: Gallium — That’s the winner. Gallium was discovered in 1875, and its properties fit Mendeleev’s predictions for eka-aluminium almost perfectly.

- Option 4: Sodium — Nope, sodium’s in a completely different column and doesn’t match aluminium’s group.

So, the correct answer is Gallium, and your answer matches that exactly.