First of all it is always a good idea to graph a question like this one. Here are the three graphs
y = x^4 - x^2 Color: Red This is the base graph
y = x^4 - x^2 - 4 Color Blue This has - 4 added to it.
y = x^4 - x^2 + 4 Color Green This has 4 added to the base graph
Which one is correct? The red and blue one are not. The graph you want is plotted upward from the base graph. The base graph (red nothing added to it) is not right because it has points on the x axis. The graph you want does not.
The same is true of the blue graph. It cuts the x axis. That only leaves the green one.
Answer: C
y = x^4 - x^2 Color: Red This is the base graph
y = x^4 - x^2 - 4 Color Blue This has - 4 added to it.
y = x^4 - x^2 + 4 Color Green This has 4 added to the base graph
Which one is correct? The red and blue one are not. The graph you want is plotted upward from the base graph. The base graph (red nothing added to it) is not right because it has points on the x axis. The graph you want does not.
The same is true of the blue graph. It cuts the x axis. That only leaves the green one.
Answer: C