I believe the correct answer from the choices listed above is option D. The statement that explains Karina's solution would be that the cost of natural gas is $0.83 per unit. Her solution was y=0.83 where y represents the cost of the natural gas. Hope this answers the question.
If you were solving a system of equations and you come to a statement like 4 = 4 ( or 0 = 0 or 1 = 1 etc ) than you know that:
D ) There are infinitely many solutions.
