Which sentence uses the word silhouettes correctly?

It would be the one about the hikers!

They can see their silhouettes on the hill (the outline of their shadows)

Hope this helps!!

The first one (At the top)

sil·hou·ette

noun

1.

the dark shape and outline of someone or something visible against a lighter background, especially in dim light:

"she paused to see its silhouette against the dimming sky"

verb

1.

cast or show (someone or something) as a dark shape and outline against a lighter background:

The mutual reward theory states that any type of relationship between two people is improved when both receive some sort of reward from said relationship. It also states that the more balanced the reward system between them is, the stronger the relationship.