Three students X, Y and Z are finding the focal length of the given concave mirror by obtaining the image of the object selected by them. X obtains the image of the grill of the nearest window of the lab. Y obtains the image of a white painted building near the lab and Z obtains a point size image of the sun. The most correct value of the focal length is obtained by

According to the NCERT text (Activity 9.2 and 9.3), the Sun is at an infinite distance from the concave mirror. The rays of light coming from an object at infinity are parallel to the principal axis and, after reflection, converge at the principal focus ($F$) of the concave mirror. Therefore, a sharp, point-sized image of the Sun is formed exactly at the focus. Measuring the distance between the mirror and this point-sized image gives the most accurate focal length.

• Why C is correct: Student Z uses the Sun (an object at infinity), so the image is formed precisely at the focus, giving the most correct focal length.
• Why A and B are incorrect: Student X (window grill) and Student Y (nearby building) use objects at finite distances. For finite object distances, the image is formed beyond the focus ($F$) (between $F$ and $C$), which would yield an incorrect, larger value for the focal length.