Answer: 4. Everyone seemed to be alone, so I decided to make a new friend that night.
Explanation:
A compound sentence is a sentence that has two independent clauses. An independent clause is a sentence that can stand on its own and needs no further clause to complete is for example; "My name is Parrain" or "Brainly helps people answer questions".
A compound sentence mixes two independent clauses with the help of a coordinating conjunction such as for, and, nor, but, or, yet, and so.
Sentence 4 is a compound sentence because there are two independent clauses which are; "Everyone seemed to be alone" and "I decided to make a new friend that night".