Answer:
It is a complex sentence.
Explanation:
There are four basic types of sentences: simple, compound, complex, and compound-complex.
A complex sentence has one independent clause joined to a dependent clause by a subordinating conjunction.
An independent clause has a subject and a predicate, and can stand alone as a sentence, as it makes complete sense. A dependent clause cannot stand alone.
In this case, "Kenneth sat down on the rickety old chair" is an example of an independent clause. "as it abruptly collapsed beneath him" is a dependent clause.
