Question

form a compound sentence.
brown pelicans
once thrived in enormous colonies along north american coasts
were dying out, but they have made a comeback
nearly died out in the eastern united states

Explanation:

To form a compound sentence, we need a clause that shows a contrast or connection. The option "were dying out, but they have made a comeback" contains a conjunction ("but") that connects two ideas, making it suitable for a compound sentence structure. The other options are independent clauses or descriptive phrases, not forming a compound structure on their own.

Answer:

were dying out, but they have made a comeback