What is one difference between fixed–rate mortgages and  variable–rate mortgages?

The answer would be Variable- rate mortgages usually start at lower interest rates than fixed-rate mortgages.

Step-by-step explanation:

First of all let us know what fixed-rate mortgages and variable-rate mortgages are.

In fixed rate mortgages interest rates are fixed when we take loan and remain same for loan's entire term and it has nothing to do with market interest rate changes.

While in variable-rate mortgages interest rates may go up and down because it changes as market interest rate change.

Upon looking at our options we can see that option B is correct because usually variable-rate mortgages start at lower interest rates than fixed-rate mortgages and goes up or down according to market interest rate change.