Hello,
B, C, and D are the correct answers. Option D is the parent function of a parabola which is a function (it passes the vertical line test). Option C is also a parabola (shifted a unit to the right) and Option B is a linear graph which is a function. Option A doesn't pass the vertical line test and therefore isn't a function. I attached an image in case you weren't sure what the vertical line test is. Hope this helps!