Verified answer

It's yellow. sec = 1/cos = 1/green (hypotenuse/adjacent; assume it's a unit circle to make it easier)

In the similar triangle that equals yellow/1

so sec = yellow/1 = yellow

They are both the same length.

They'll both be sec(α) since the circle is clearly the unit circle.

1/yellow = cosα

yellow = secα

Considering the triangle involving the black line and the part-yellow line (ie yellow until the red line), the angle opposite the part yellow = π/2 - α

So sin(π/2 - α) = part yellow / black = 1/black

cosα = 1/black

secα = black

all of those trig functions are RATIOS. So sin is red/yellow, etc. So what's the question?