Which of the following metals can be used to reduce compounds of iron to iron metal?

A more reactive metal can displace a less reactive metal.

Among the four options, Al, Mn and Na are more reactive than iron, but Hg is less reactive than iron.
Hence, Al, Mn and Na can be used to reduced compounds of iron to iron metal.
However, Na is a very reactive metal which reacts vigorously with water. If Na is used, the reaction should not take place in aqueous solution.

The answers: A, B and D

Reducing iron(II) to iron metal....

Look at the activity series. Metals above iron will reduce iron ions to the metal as they, themselves, are oxidized. Since mercury is below iron, we can eliminate that one immediately.

Sodium, aluminum and manganese are above iron and so theoretically they would work. Or will they?

Sodium metal reacts vigorously with water to make a solution of NaOH and liberate hydrogen gas. Therefore, sodium will react preferentially with the much more abundant water in the solution and not so much with iron(II) ions. So you can eliminate sodium.

Aluminum metal is always covered by a passivating layer of aluminum oxide, Al2O3, which prevents it from reacting. You could force it to react by adding a source of chloride ion which will react with Al2O3 to make AlCl4^- and expose the aluminum metal to react with Fe(II) ions. But if you didn't do that, then aluminum metal would do nothing.

That leaves you with manganese as the only viable choice.

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There is another pathway for a spectacular reaction: the thermite reaction. It involves powdered aluminum mixed with powdered iron(III) oxide. This reaction produces a great deal of heat and molten iron. It can be very dangerous and many precautions must be followed.