Answer:
The expression that belongs in the box is 8 x 4
Option C is correct answer.
Explanation:
We need to find Which expression belongs in the box? 

The property applied here is associative property of multiplication.
Associative property of multiplication states that:

So, if we apply the property to our question we have:
a = 3
b = 8 
c = 4
So, the expression will become: 

So, the expression that belongs in the box is 8 x 4
Option C is correct answer.