Final answer:
A tree's shadow is longest when the Sun is about to go down, because the sun is low on the horizon and casts extended shadows.
Step-by-step explanation:
The longest shadow that a tree casts occurs when the Sun is nearest to the horizon, which is during early morning after sunrise and late afternoon before sunset. At these times, the sun is at a low angle in the sky, which results in longer shadows. Therefore, the correct answer to the question 'Which time of day would a tree's shadow be the longest?' is A. when the Sun is about to go down. Conversely, when the Sun is at its highest point in the sky, or noon, the shadows are at their shortest.