just look where the functions are undefined or have cusps. for (a), it is not differentiable anywhere sin(x) = 0. and that is at infinitely many points (not including 1/2 pi and -1/2 pi), but rather, at k*pi, for k an integer

for (b), the logarithm is undefined at zero, and hence, is not differentiable there

try (c)

if you want to be sure of these answers, try using the limit definition of a derivative, the limit will not exist where the function is not differentiable