What is the slope of the line on the graph?

The slope of this line is -1/3. To determine slope, you can remember the phrase rise over run. To elaborate, this example has points on (0,4) and (6,2). Use rise over run to solve this.
Starting from the (6,2) go up two units to be on the same line which (0,4) is on. Count the units going left until you reach that point. You should have come up with 6. Technically, the slope would be -2/6 because the line is going downwards, but it can be simplified to -1/3.
An actual equation you can use is y2-y1/x2-x1: 6-4/0-3= -2/6 = -1/3

Cbottom left is the correct one.