In a right triangle, the tangent of an angle is defined as the ratio of the length of the opposite side to the length of the adjacent side. Here, we are looking for the length of side c, which is opposite the 75° angle. The adjacent side is represented by the blank space. According to the definition of tangent, we can express this relationship as tan(75°) = c / (length of adjacent side). Rearranging this equation gives us c = (length of adjacent side) * tan(75°). Since the length of the adjacent side is given as 25, we substitute this value into the equation, resulting in c = 25tan(75°). Therefore, the correct answer is C. 25tan75°.