AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Option A is correct. Both A and R are true, and R is the correct explanation of A. Under the New Cartesian Sign Convention, distances measured to the left of the pole (in front of the mirror) are negative. Since the principal focus of a concave mirror lies in front of the mirror, its focal length is negative.
The key is linking the sign convention rule (left of pole = negative) directly to the position of the principal focus of a concave mirror (in front = left side). R correctly and completely explains why f is negative for a concave mirror, making Option A the right choice.