Final answer:
The exact value of tan²x is sin²x/cos²x as per the definition of the tangent function.
Step-by-step explanation:
The exact value of tan²x, where x is an angle, is given by the trigonometric identity tan²x = sin²x/cos²x. This is because tanx is defined as sinx/cosx, and squaring both sides gives us tan²x = (sinx/cosx)² = sin²x/cos²x. Hence, the correct answer to the question is c. sin²x/cos²x.