Parallel rays, from a distant building, incident on the concave mirror, form its distinct image on a screen as shown. A student was asked to find its focal length with the help of this experiment. Correct measure will be :

When parallel beam of light rays incident on a concave mirror after reflection all the reflected rays are convergent focus so in the given diagram pole of the mirror is  at 1 cm and focus of the mirror is at 6.4 cm. the distance between pole to focus is focal length

$\text{f}=6.4-1=5.4\text{cm}$