Answer:
Complex sentence.
Step-by-step explanation:
A complex sentence contains one independent clause and one or more dependant clauses.
An independent clause can stand on its own. The information in it makes sense and does not need extra information to understand its meaning.
A dependant clause can not stand on its own. If we read a dependant clause alone, some information is missing, and we cannot fully understand the meaning of this clause.
In this case, the independent clause is -It abruptly collapsed beneath him- and the dependant clause is - As Jim sat down on the rickety old chair-. We can see that the dependant clause is missing information and that it contains the word as, which is a subordinating conjunction that links the clauses and is present in dependant clauses.