which of the following can be used to find the focal lenght of convex lens ?

object at a distance of 10cm for lens of focal lenght 30cm

light from a laboratory window

light from sun  

light from a tree next to the window

Correct option Light from sun as because the sun is far away and so the rays from sun are parallel so the image will be formed at the focus.

Object at 10 cm for lens of focal length 30 cm will form a virtual image and thus focus cannot be judge accurately.

Light from nearby tree are not parallel and image wont form at focus, so focus cannot be measured.

Rays from laboratory window may not be parallel also. So it cant be the correct option as well.