Explanation:

- The core of the Earth is primarily made up of nickel and iron.

- Option 3: Nickel and Iron - This is the correct option. The Earth's inner core is solid and largely consists of iron and nickel, which is why it's very dense.

- Option 1: Nickel and Silica - Silica is not a major component of the Earth's core. It's more associated with the Earth's crust.

- Option 2: Silica and Magnesium - These elements are more prevalent in the Earth's mantle rather than the core.

- Option 4: Silica and Alumina - These are components found in the Earth's crust.

The correct answer is Option 3: Nickel and Iron.