To solve this question, the trigonometric identity sin x = cos (90-x) is required. It can also be written as cos x = sin (90-x). It can be seen that this identity holds when the two angles are complementary i.e. they sum up to 90 degrees. Therefore, the answer can be determined by substituting all the options one by one in the identity cos x = sin (20+x). If x=30 degrees, then x+20=50 degrees. 30 and 50 are not complementary. If x=35 degrees, then x+20=55 degrees. 35 and 55 are complementary since their sum is 90 degrees. Therefore, B is the correct choice!!!
