Which of the following pair of compound name - formula is correct?

I. Calcium oxide - $Ca_2O$

II. Aluminum Oxide - $Al_2O_3$

Only II

The correct answer is option 4. Only II.

Let us break down the explanation for each compound and its formula:

I. Calcium Oxide - Ca₂O:

The correct formula for calcium oxide is \(CaO\), not \(Ca₂O\). Calcium oxide is composed of one calcium (Ca) atom and one oxygen (O) atom, resulting in the formula \(CaO\). Therefore, Option I is incorrect.

II. Aluminum Oxide - Al₂O₃:

The correct formula for aluminum oxide is \(Al₂O₃\). Aluminum oxide is composed of two aluminum (Al) atoms and three oxygen (O) atoms, resulting in the formula \(Al₂O₃\). Therefore, Option II is correct.

In summary:

Calcium oxide has the formula \(CaO\) (Option I is incorrect).
Aluminum oxide has the formula \(Al₂O₃\) (Option II is correct).

Therefore, the correct answer is option 4. Only II.