The minimum height of the box must be 18 units, which is present in all the choices. Another minimum dimension that we already know is 7 units, since it is an oblique box, and the projection of bottom-left to top-right is shown as 7 units. That leaves us with the second (7x6) or fourth choice (7x3).
We have not been given the other dimension of the triangle, but from common sense, it looks longer than the front (5 units) leg, therefore 6 units is not an unreasonable dimension.
With 99.5% certainty, I would choose the second choice (7x6x18) because the remaining dimension does not look like 3 units long.
