The vertical asymptotes occur when θ is an odd multiple of π/2 (including negatives) but the tangent of multiples of π (that is, even multiples of π/2) is zero (because sin(nπ)=0 and tan(θ)=sinθ/cosθ). The asymptote occurs when cosθ=0, that is, when θ is an odd multiple of π/2 which would result in dividing by zero to get tanθ. So option b applies because tan(0)=sin(0)=0.