The process of using sacrificial anodes to protect iron from rusting is known as cathodic protection. In this process, a more reactive metal is connected to the iron object, which acts as the anode and sacrifices itself to prevent corrosion of the iron, which is the cathode. The sacrificial anode corrodes instead of the iron, effectively protecting it.

In the given options, all the metals - zinc, magnesium, and aluminum - can be used as sacrificial anodes to protect iron. These metals are more reactive than iron and have a higher potential for oxidation, making them effective in preventing rust formation.

However, tin cannot be used as a sacrificial anode. Tin is less reactive than iron and has a lower potential for oxidation. Therefore, it would not be able to sufficiently protect the iron from rusting.

In conclusion, the correct answer is option 1 - Tin cannot be used as a sacrificial anode for cathodic protection.