Answer:
1. The trees hovered over the hiking trails like <em><u>an umbrella</u></em>.
2. The lightning flashed like <u><em>Zeus' thunderbolt</em></u>.
3. His heart was as cold as <em><u>stone</u></em>.
4. The car was as fast as <em><u>the Flash</u></em>.
5. The ocean was as blue as <u><em>a clear sky</em></u>.
Explanation:
Similes are literary techniques used to make comparisons between similar things though there is no relation between the two. They are the same as metaphors except that similes use "like" and "as" in each comparison.
The given incomplete sentences are filled with similes as follows-
1. The trees hovered over the hiking trails like <em><u>an umbrella</u></em>.
2. The lightning flashed like <u><em>Zeus' thunderbolt</em></u>.
3. His heart was as cold as <em><u>stone</u></em>.
4. The car was as fast as <em><u>the Flash</u></em>.
5. The ocean was as blue as <u><em>a clear sky</em></u>.