The A is a function, because all the values of the independent variable (the first value of the ordered pairs) are related to only one value, i.e. have only one image.
In B - 2 has two different images: 3 and 1. That makes that this is not a function.
In C, 3 has two different images: 5 and 2. That makes that this is not a function.
In D, - 1 has two different images: 1 and 5. That makes that this is not a function.
