Graph-A and graph-D could both accurately show the radioactivity of the sample as the weeks go by. The only difference is that 'A' had 20 units of radioactivity when the time began and 'D' had 40 units. But on both of them, any point on the graph is 1/2 the value of what it was a week earlier ... which is exactly what you expect.
The equation under graph-A is the correct equation of that graph. We don't know anything about the equation under graph-D, because it sank below the bottom of the picture and we can't see it.
The equation of graph-D is
R = 40 (1/2)^x .
If that's not what it says under graph-D, then D is wrong, and only A is a correct answer.
