This is the associative property, since any numbers can <em>associate </em>with each other, via the parentheses.
Answer:
<u>The</u><u> </u><u>fourth</u><u> </u><u>number</u><u> </u><u>line</u><u> </u><u>is</u><u> </u><u>the</u><u> </u><u>answer</u><u>.</u>
Step-by-step explanation:

We subsituete
9:6=6:4
treat as fraction
9/6=6/4
9/6=3/3 times 3/2
6/4=2/2 times 3/2
3/2=3/2
this is TRUE
The answer is the first option. 7 left is not - 7 in this case. With these questions, you switch the signs. The vertical movement signs stay the same. Therefore, the answer is the first option.