Another way to say constant rate of change is slope  of a linear line.  
Notice that in answer (1), while x is getting bigger, so is y — this is a positive slope of 3, and definitely does not match with the -3 we are looking for.
If you graph answer 2, it’s all over the place and doesn’t even have a constant rate of change.
Looking at the graph in answer 3, we can either use rise over run or the slope formula 
m = (y2-y1)/(x2-x1) using two points on the graph (0,3) and (1,1).  We can also say “When x  goes up by 1, how much does y change?” and here we can see the slope is -2.
For answer 4, we would divide both sides by 2 to get a y= mx + b which would be y = -3x + 5 and here we can see the slope (m) would be -3.