Which of the following is a complex sentence? a. mark wanted blueberry pie and vanilla ice cream. b. eileen bought pants but not a new jacket.

B. Eileen bought pants but not a new jacket

Explanation:

Complex sentences are sentences that have a dependant and independent clause.

An independent clause is a part of a sentence that even when you separate it from the rest of the words, you can read a complete idea: "Eileen bought pants"

A dependent clause is a part of the sentence that if you separate it from the rest of the words it reads incomplete.  "But not a new Jacket" If we would be reading only this part of the sentence, we would realize that there is something missing before the word "But", this is so, because it is a dependent clause, it needs the other part of the sentence so that the idea is complete.