Answer:
D. y = cot x
look at the picture.
cot(x) not exist for x = kπ, k∈Z
(vertical asymptotes ...,-2π, -π, 0, π, 2π, ...)
tan(x) not exist for x = π/2 + kπ, k∈Z
(vertical asymptotes ...,-3π/2, -π/2, π/2, 3π/2, ...)
csc(x) not exist for x = kπ, k∈Z
(vertical asymptotes ...,-2π, -π, 0, π, 2π, ...)
cos(x) exist for any value of x