Hint: It is mentioned in the question the substance is made of pieces of iron. The answer is also a word that is usually prefixed to things that avoid any randomness on unusual happening. The answer can be approached by eliminating the other options.
Let us first understand the terms in the options one by one,
Preservatives are substances usually used to store things having a chemical background. But iron is very reactive to any chemical and hence cannot be a preservative
Hence the answer is no preservative.
Speaking about the compass and poles they themselves are part of magnets, hence the correct answer is keepers.
Additional Information:
Keepers are small pieces of the soft iron core, placed across the poles of the magnet to help preserve the strength of the magnet by completing a magnetic circuit. A keeper protects the magnet from getting attracted to other pieces of metal as well.
A magnet usually consists of small domains the size of a few microns that have their own magnetic field. When all these domains are aligned in a particular direction their magnetic components get added up and form a strong magnet. Magnet with low coercivities (the ability of a magnetic substance not to get demagnetized) usually loses its magnetism due to the earth’s magnetic field or some field produced by a current-carrying wire. A keeper already keeps the domains aligned and prevents magnets from losing their strength.
Given below is a diagram of how magnets are kept using keepers. In Fig 1 the two bar magnets are kept such that the keepers on either side complete the magnetic circuit. In Fig 2 a single keeper is used to complete the magnetic circuit in horseshoe magnet.
Note: Not all magnets are stored using magnet keepers. Magnets such as super magnets have so strong pole strength that they cannot get demagnetized under the influence of the external magnetic field. These magnets are usually kept at lower temperatures.
