(A)

Suppose the displacement of the first block is x_{1} ànd that of second is x_{2}, then

X=(mx1+mx2)/2m or Ft²/4m=(x1+x2)/2

So x1+x2=Ft²/2m......(1)

And, the extension of the spring is x1-x2=x0........(2)

From 1 and 2

X1=1/2[(Ft²/2m)+x0] and - for x2