Which of the following components of an ARM adjusts periodically?

In an Adjustable-Rate Mortgage (ARM), the index is a benchmark interest rate that can change periodically, and the interest rate on the loan adjusts based on changes to this index, plus a fixed margin. The margin itself remains fixed throughout the life of the loan.

“The interest rate on an ARM is composed of two parts: the index (which fluctuates) and the margin (which is fixed). The interest rate adjusts periodically based on changes in the index.”

— SAFE MLO National Test Study Guide; CFPB’s Consumer Handbook on ARMs