Let f (x)= |cosx|. Then :

f is everywhere differentiable. f is everywhere continuous but not differentiable at n = nπ, n∈Z. f is everywhere continuous but not differentiable at x = (2n + 1) π/2, n∈ Z . None of these.

f is everywhere continuous but not differentiable at x = (2n + 1) π/2, n∈ Z .

Try to differentiate |cos x|, you'll see it is not-differential.