Step 1: The cornerstone of iodine deficiency disorder control is universal salt iodization, that is fortification of salt with iodine. So option c is correct.
Step 2: Iodized salt is the most economical, convenient and effective means of mass prophylaxis in endemic areas because salt is consumed by almost everyone in fairly constant amounts throughout the year.
Step 3: Under the National Iodine Deficiency Disorders Control Programme, salt is fortified at the production level so that adequate iodine reaches the consumer despite losses during storage and cooking.
Step 4: Health education (option a) supports the programme but is not the primary control measure, and water testing (option b) is irrelevant to delivering iodine, so they are not the best answer.