Explanation

With t = cos79°, cosec79°(1 − cos79°) becomes (1 − t)/sin79°. Since sin79° = √(1 − cos²79°) = √(1 − t²) = √((1−t)(1+t)), the fraction simplifies to √((1 − t)/(1 + t)). This is the form printed as option (d), the radical being part of the option as printed in the paper.