(D) It has been made mandatory for all parties to conduct organizational elections and file income tax returns.
Source: How can parties be reformed?, Chapter 4
Option (A) is wrong — the Constitution amendment to prevent defection was done by Parliament, not the Election Commission. Option (B) and (C) refer to a Supreme Court order, not an Election Commission step. Only option (D) correctly identifies an Election Commission order. Students often confuse the three agencies (Parliament, Supreme Court, Election Commission) involved in different reform measures.