Answer:
option A
Step-by-step explanation:

Since they are right triangles use Pythagorean Theorem.
a^2 + b^2 = c^2 where c is the hypotenuse.
a. 12^2 + b^2 = 13^2
144 + b^2 = 169
subtract 144 from both sides
b^2 = 25
take the square root of both sides
t = 5 This is also know as a Pythagorean Triple 5-12-13
b. a^2 + 9^2 = 12^2
a^2 + 81 = 144
subtract 81 from both sides
a^2 = 63
take the square root of both sides
a = √63
a = √(9 * 7)
a = 3√7
c. 6^2 + 9^2 = c^2
36 + 81 = c^2
117 = c^2
take the square root of each side
c = √117
c = √(9*13)
x = 3√13
4 loads of stone ... 2/3 ton
1 load of stone ... x ton = ?
If you would like to know the weight of 1 load of stone, you can calculate this using the following steps:
4 * x = 2/3 * 1
4 * x = 2/3 /4
x = 2/3 / 4
x = 2/12 = 1/6 ton
The correct result would be 1/6 ton.