ANS:A - avoid its decomposition.

DDT should not be allowed to come in contact with iron (during its manufacture) to prevent sulphonation of the monochlorobenzene by the acid catalyst.

Explanation:

1. Manufacturing Process:
   • DDT (Dichloro-Diphenyl-Trichloroethane) is synthesized from chloral (trichloroacetaldehyde) and chlorobenzene in the presence of a strong acid catalyst, typically sulfuric acid.
   • During this process, monochlorobenzene is one of the intermediates formed.
2. Role of Iron:
   • Iron can act as a catalyst for undesired side reactions, particularly the sulfonation of monochlorobenzene. Sulfonation involves the addition of a sulfonic acid group (SO₃H) to the benzene ring, which alters the chemical structure and properties of the compound.
   • This sulfonation reaction is not part of the intended synthesis pathway for DDT and can lead to the formation of unwanted by-products.
3. Precautionary Measures:
   • To ensure the purity and yield of DDT, it is crucial to prevent any contact between iron and the reaction mixture. This is typically achieved by using equipment and materials that are resistant to corrosion and not reactive with the acid catalyst.

Conclusion:

Therefore, the statement "DDT should not be allowed to come in contact with iron (during its manufacture) to prevent sulphonation of the monochlorobenzene by the acid catalyst" correctly identifies a critical aspect of ensuring the proper synthesis of DDT without unwanted chemical reactions that could affect its quality and purity.