Cruciferous plants when crushed emit a pungent smell. It is due to the presence of

Cruciferous vegetables are rich in nutrients, including several carotenoids, vitamins C, E, and K, folate and minerals. They also are a good fiber source. In addition, cruciferous vegetables contain a group of substances known as glucosinolates, which are sulfur-containing chemicals. These chemicals are responsible for the pungent aroma and bitter flavor of cruciferous vegetables. During food preparation, chewing, and digestion, the glucosinolates in cruciferous vegetables are broken down to form biologically active compounds such as indoles, nitriles, thiocyanates, and isothiocyanates. Indole-3-carbinol and sulforaphane have been most frequently examined for their anticancer effects. So, cruciferous plants when crushed emit a pungent smell. It is due to the presence of sulphar compounds and not due to presence of oxalic acid, alkaloids or iron compounds. 
Therefore, the correct answer is option D.