A function is a graph where every input (x-value) has one and only one output (y-value). In other words if the value for x=2, there will only be one corresponding point for y. For the function in B, that y-value would be 4. Both A and D, solve for y, means taking the square root which leaves a +/- x-value. C is absolute value of y, which means the positive and negative values for y would have the same point. Only choice B has x-values corresponding to a singular y-value. If you can remember what these graphs look like you can test if they are functions based on the vertical line test.
