Explanation:

- The oceanic crust is primarily composed of silica and magnesium, often referred to as sima.

- It is generally denser and thinner than the continental crust.

- Option 1: Silica and magnesium

- This is correct.

- Oceanic crust typically consists of basalt, which is rich in silica (SiO2) and magnesium.

- Option 2: Silica and alumina

- Silica and alumina (sial) are more characteristic of continental crust than oceanic.

- Option 3: Magnesium and nickel

- Magnesium and nickel are more common in the Earth's mantle and core.

- Option 4: Iron and alumina

- Iron and alumina pairing doesn't accurately represent the oceanic or continental crust.