- Article 249 of the Indian Constitution empowers the Parliament to legislate on a subject enumerated in the State List if the Rajya Sabha passes a resolution by a two-thirds majority of members present and voting, declaring that it is necessary or expedient in the national interest. Such a resolution remains in force for a period not exceeding one year but can be renewed. - Other conditions under which the Union Legislature can legislate on a State List subject include during a National Emergency (Article 250), when states request Parliament to do so (Article 252), or for implementing international treaties or agreements (Article 253). - The question asks "only when", which is technically inaccurate as there are multiple scenarios. However, among the given options, the Rajya Sabha resolution (Article 249) is a specific mechanism for Parliament to legislate on a state subject in national interest, distinct from the broad powers during an emergency (D), and not dependent on Presidential order (A) or Speaker's permission (C). Option B represents a direct legislative trigger within the federal structure.