Which of the following is a trigonometric identity?
Correct: B
An identity is an equation that is true for all permissible values of the variable.
(A) sin θ = cos θ is only true for specific angles (e.g., θ = 45°), not for all θ.
(B) sin²θ + cos²θ = 1 is a fundamental trigonometric identity, true for all values of θ.
(C) tan θ = 1 is only true for specific angles (e.g., θ = 45°).
(D) sin θ = 1/2 is only true for specific angles (e.g., θ = 30°).
Therefore, sin²θ + cos²θ = 1 is the correct identity.