Stars emit their own light so they twinkle but planets just reflect the light falling on them by stars so they don't twinkle.
awelllitdistanttree is the answer.
to find the focal length we have to measure the distance between the lens and the screen, is the distance between the principal focus and optical centre of the lens. in order to form an image at the principal focus, the object has to be placed at infinity. to get a clear image, the object should be well illuminated. therefore a well lit distant tree is the most appropriate option.mark my answer brainliest if it you.