How does adjustable rate mortgages work? What are the three numbers X/X/X on ARMs? Can someone help me with this question about adjustable rate mortgages?

Matt has obtained a 30-year 3/1 ARM mortgage loan for $150,000 on a beautiful four-bedroom two bath home in the suburbs. The start rate was 3%. The rate caps are 4/2/8. The margin is 3%. The index was 4% at the start of the loan, 4.25% at the end of year three, and 6.5% at the end of year four. Which of the following rates would reflect the interest rate that will begin in year number five?