Given that on 18th April 1603 is Thursday. What was the day on 18th April 2003?

Answer: Option C

After every 400 years, the same day occurs. (Because a period of 400 years has 0 odd days)
Thus, 18^th April 1603 is Thursday, After 400 years i.e., on 18^th April 2003 has to be Thursday.