The slope of a position vs. time graph tells you the velocity of the object in motion.
Slope formula: (y2-y1)/(x2-x1)
Let (x1,y1) =(0,7) and (x2,y2)=(7,0)
Plug these values into the formula.
(y2-y1)/(x2-x1) => (0-7)/(7-0) => -7/7
=-1 m/s
Thus, the last option is correct.