The correct option is A 10 feet
The sum of any two sides of triangle should be greater than the third side.
If we consider the first option, this condition holds as shown below:
5 feet + 7 feet = 12 feet > 10 feet
5 feet + 10 feet = 15 feet > 7 feet
10 feet + 7 feet = 17 feet > 5 feet
But, if we consider the triplet 5 feet, 7 feet and 12 feet, since
5 feet + 7 feet = 12 feet ≯ 12 feet.
Similarly, we can rule out the other options.