Suppose you are trying the test hypothesis a. if you have shown that hypothesis a is true for one case, have you shown that hypothesis a is correct? explain your reasoning.

You could give an example or research into previous hypothesis and tests for the topic you are on.
You could also link your hypothesis to something opposite. (Eg: If you were looking at ice melting you may want to also look at how it freezes, this would show that even though something happens it reverses to its original state, in this case water.)

C. expression because any expression or equation may contain more than one variable

The correct graph is the second one, with the points being at (-6,0), (2,0) and reaching down to (-2,-8)