first, you must combine the amount of miles walked so you can figure out how much she walked in all. to do that, you have to find like denominators.
3/4 = 9/12
7/12 = 7/12
9/12 + 7/12 = 16/12
16/12 = 1 4/12 = 1 1/3
in total, she walked 1 1/3 mile, which is the third option :)