The function \( f(x) = |x| \) is not differentiable at \( x = 0 \) because the left-hand and right-hand derivatives do not match.
The left-hand derivative is \( -1 \) (approaching from the negative side).
The right-hand derivative is \( 1 \) (approaching from the positive side).
Since the derivatives do not match, the derivative at \( x = 0 \) does not exist.
Conclusion.
Thus, the correct answer is (d) None of these.