Which of the following equations describe the condition for observing the fourth dark fringe in an interference pattern correctly?

Question

d sinθ = λ

d sinθ = 4λ

d sinθ = 5λ/2

d sinθ = 7λ/2

I would think B but I feel like the answer that has a '4' in it (fourth fringe, four... yeah) would be too obvious

Answer 1

d•sinθ=2k•λ/2=k λ
k=4
d•sinθ==4 λ