Answer
$m = 0.25~kg$
Work Step by Step
On the graph, we can see that the acceleration is $a = 4~m/s^2$ when the force is $F = 1.0~N$. We can use this information to find the mass:
$F = ma$
$m = \frac{F}{a}$
$m = \frac{1.0~N}{4~m/s^2}$
$m = 0.25~kg$
