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The correct answer is B.
A mortgage that starts out with a fixed rate and converted to a fixed rate later is referred to as a rollover or renegotiable mortgage. These are often used in Canada, Germany, Denmark, etc.
If the mortgage starts at a fixed rate and is later converted into an adjustable rate mortgage, then we would say this is a hybrid mortgage.
A convertible mortgage is an adjustable-rate loan that gives the borrower the option to convert the loan to a fixed-rate mortgage.