The area of triangle M'N'O' is 49x square units.
This is because when a figure is dilated by a scale factor of 7, the area is scaled by the square of the scale factor. In this case, the scale factor is 7, so the area is scaled by 7^2, which is 49. Multiplying 49 by x gives the area of triangle M'N'O'.
Therefore, the correct answer is B. (7)^2x square units.