Identify the concept

This is an application of electrochemistry, specifically cathodic protection.

What happens to a ship in seawater?

The body of a ship is usually made of iron or steel. In the presence of water and dissolved salts, corrosion (rusting) can occur.

Why is magnesium attached?

Magnesium is more reactive than iron. Its standard reduction potential is much more negative, so magnesium gets oxidized more easily:

$\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-$

Thus, magnesium acts as a sacrificial anode.

Effect on the ship

Because magnesium oxidizes in preference to iron, the ship's iron body becomes protected from oxidation. So the action of water and salt on the ship is reduced/prevented.

Evaluate the options

• A: make the ship lighter — Incorrect. Adding magnesium blocks does not make the ship lighter.
• B: prevent action of water and salt — Correct. More precisely, magnesium protects the ship from corrosion caused by seawater.
• C: prevent puncturing by under-sea rocks — Incorrect. Magnesium blocks are not for mechanical protection.
• D: keep away the sharks — Incorrect. No such purpose.

Final answer

The correct option is:

$\boxed{\text{B}}$

Comparison with stored answer

Stored correct answer: B

My derived answer: B

Hence, the answers agree.