The image shows a ray of light bending at the boundary of a glass block. according to a law of optics, = n, where n is a constant. which equation states the same law in terms of a and b?

The formula for the light bend would be n1*sin i= n2 * sin r

If you use "sin (90-x)= cos x" principle, you can change the sin in the equation into
n1*sin i= n2 * sin r
n1 * cos a= n2 * cos b
n2/n1= cos a/ cos b

Assuming the n constant is the ratio of the glass compared to water then the answer would be: cos a/ cos b