a well-illuminated distant building
well-lit grills of the nearest window
a distant tree or an electric pole
a burning candle placed at the distant edge of the laboratory table
The student needs a parallel beam of light falling on the concave mirror so that after reflection the reflected rays meet at a point indicating the position of the focus. We know that the light rays which are coming from a distant object of smaller (comparatively) size are parallel to each other and parallel to the principal axis as well. Hence the student should choose a distant tree or an electric pole. The image hence formed will be the location of the focus. Hence correct answer is option A.