In a right triangle, one acute angle measures 35°. If (35°) = (x°), what is the value of x?

Sine and cosine are cofunctions: the sine of an angle equals the cosine of its complement. So $\sin(35°) = \cos(90° - 35°) = \cos(55°)$, and $x = 55$, choice C. In a right triangle, the two acute angles add to $90°$, and the side opposite one angle is adjacent to the other, which is why this works. Choice A (35) would say $\sin 35° = \cos 35°$, which is false, since those are different values. Choice B (90) is the sum, not the complement. Choice D (65) is $90° - 25°$ or $180° - 115°$, and has no basis.