Which of the following functions is not differentiable in the domain $[-1,1]$ ?
(a) $f(x) = x^2$
(b) $f(x) = x-1$
(c) $f(x) = 2$
(d) $f(x) = Maximum (x,-x)$
$x^2$ is differentiable and $x-1$ is continuous and differentiable in the given domain.. make the graph of both and check..
Actually $|x|$=maximum(x,-x) , you can verify this here https://math.stackexchange.com/questions/1856255/prove-x-max-x-x
and we know that $|x|$ is not differentiable at x=0 which is in interval [-1,1] .
Rest of the functions are continuous and differentiable everywhere.
so option D.