k = (1+sina)(1+sinb)(1+sinc) = (1-sina)(1-sinb)(1-sinc)
Multiply both entities, k x k = (1+sina)(1+sinb)(1+sinc) x (1-sina)(1-sinb)(1-sinc)
That gives k^2 = (1 - sin^2a)(1 - sin^2b)(1 - sin^2c)
k^2 = (cos^2a) x (cos^2b) x (cos^2c)
k = sqrt ((cos^2a) x (cos^2b) x (cos^2c))
k = cosacosbcosc
option a is correct.
