The length of the shadow will be longest when the
A. Angle between the source of light and the object is $45$
B. Source of light and the object are parallel to each other
C. Source of light and the object are perpendicular to each other
D. Angle between the source of light and the object is $60$

Hint: Recall the law of reflection. The longest shadow that can be formed by a source is an infinitely long shadow. Recall that the shortest shadow on the surface is formed when the Sun is directly over the head.

Complete answer:
-Sunrise and sunsets produce the longest shadows of the day, because that’s the most extreme angle possible before the horizon blocks the sun. So, in real life, the longest shadow we can observe is during the sun rise or the sunset.
-Noon will produce the shortest shadow of the day, though this length will vary based on the time of the year and how far from the equator the object is. The further away from the equator we are, the longer the shadow is produced, because the sun will never reach directly overhead.
-It is clear from the above discussion; the longest shadow will be produced when the angle between the source and the object is ${90^ \circ }$ . That is, when the source of light and the object are perpendicular to each other.

Therefore, option C is the correct option.

Note: Remember that the shortest shadow is formed when the source is parallel to the object. And the longest shadow is formed when the object and source are perpendicular to each other. Also, the length of shadow increases as the angle increases from ${0^ \circ }\,to\,{90^ \circ }$ and then the length of the shadow decreases. And then at ${180^ \circ }$ the length of the shadow is again minimum.