C,D and F are correct.
C is what you would think of when reading it, you walked 12 and walked 1/4(12) more.
12 + 12(1/4) =
12 + 3 =
15
D is equal to C when simplified,
12(1+1/4) =
12 + 3 =
15
F is the same as the D, it just simplified the numbers in the parenthesis
12(1+1/4) =
12(4/4 + 1/4) =
12(5/4) =
15