A:

I think the answer is

2

because

3/2 is the ratio of height to width, for a rectangle
and since the tank stand extends the width, it would have a width of 1
if you can fill the box by 2 it has to be at least 2 tall

and that

(with the width of the tank stand and a depth of 3 of the trench) gives 2 : 3 : 6 = 6

thus the answer must be

6

because

6 can be interpreted as (1+2) = 3, so 2 becomes the ratio of (1+2):(3+6) = 5:7

or

6 is the number of tank stands of the trench

and since

filling the box with 6 would require a width of 5 and a length of 7 (because each box would require a 1:6 ratio)

3 : 6 = 5 : 7 is the actual ratio of the width to the length.

A:

The answer is 6 because...

A 6 foot tank can only fit through the 3-inch trench, which equals 6 feet.

The other answers were correct but I feel this one is the shortest and clearest.