Question

if a leaf is too far away, the ants crawl over each other. each ant grabs the middle of another ant. they hold each other up. it is an ant chain!
more ants go across the chain to get to the next leaf. then the ants pull the two leaves together.
the ants are making an ant chain.
why do ants build an ant chain?
so they can get stronger
so they can reach leaves
so they can climb a tree

Explanation:

The text states that if a leaf is too far away, ants build an ant chain. More ants go across the chain to reach the next leaf. So the purpose is related to reaching leaves.

Answer:

so they can reach leaves