You should be able to answer c without doing any computation at all. As $1.41$ is between the upper and lower cost elements there is some mix that will produce it, so d is out. If you had only two things you could mix you could find a unique mix that would produce that cost. Given that you have three, there is a one dimensional set of solutions, so there will be some other than a and b. If you trust the problem setter to have made a unique solution, neither a nor b should yield the correct cost. You could check if you want.