The graph of the relationship between the sumo wrestler's weight and the number of months on the diet would be a straight line with a positive slope.
This is because the sumo wrestler's weight increases at a constant rate of 5.5 kilograms per month. As a result, the graph of the relationship between the wrestler's weight and the number of months on the diet would be a line with a slope of 5.5 that passes through the point (0, 79.5) on the coordinate plane.
Option (C) appears to be the graph that best matches this description. Option (A) has a negative slope, which does not match the given information. Option (B) has a positive slope, but it does not pass through the point (0, 79.5). Option (D) has a positive slope and passes through the point (0, 79.5), but it appears to have a larger slope than 5.5, which does not match the given information.
Therefore, the correct answer is (C).