How would you describe the slope of this graph ?

I would describe it as the second choice, B. negative.
The easy way to find the answer is to look at the line from left to right. If the line is going down from left to right, then it is negative. If it is going up, then it is positive. If it goes from left to right in a straight line, with no slope (doesn't go up or down), then it is 0. If the line does not go left or right, and is a simple line going from the top of the graph to the bottom, then it is undefined.
Hope this helps :-)