1) Identify the most abundant greenhouse gas.
It seems to be Carbon dioxide in your chart
2) State one human activity that is the source of this gas.
Try, The gas that the vehicles spew out while moving (Is it good enough?)
3) State one negative effect of greenhouse gases in the earths atmosphere.
Too much of any greenhouse gases could lead to our planet warming up or Global Warming.
Just as the saying goes, " Too much of a good thing is a bad thing"
Hope this helped :)