Let S be the set of all points in (-π, π) at which the function f(x) = min(sin x, cos x) is not differentiable. Then S is a subset of which of the following?

Let f(x) = min(sin x, cos x).
The function f(x) is not differentiable at points where sin x = cos x.
This occurs when tan x = 1, which means x = π/4 + nπ, where n is an integer.
In the interval (-π, π), the solutions are x = -3π/4, -π/4, π/4, 3π/4.
However, since we are considering the interval (-π, π), we only need to check the points in this interval.
The points where sin x = cos x in the interval (-π, π) are -3π/4, -π/4, π/4, 3π/4.
However, only -π/4 and π/4 are in the interval (-π, π).
Therefore, S = {-π/4, π/4}.
The set S is a subset of {-π/4, 0, π/4} and {-π/2, -π/4, π/4, π/2}.
The correct option is {-π/4, 0, π/4}.