Which is a run-on sentence? a. her book report is due thursday she hasn't read the book yet. b. her piano lesson starts at 4 o'clock, and she's going to be late. c. her closet is full of clothes, yet she says she has nothing to wear. d. her cookies are delicious, so she entered them in the county fair.

A. Her book report is due Thursday she hasn't read the book yet.

is the run-on sentence.

This is because a run-on sentence is a sentence in which two complete thoughts are trying to be conveyed, without proper use of punctuation and grammar.  

In this case, "Her book report is due Thursday" and "She hasn't read the book yet", should probably be two separate sentences.  Or, they could be linked with a comma, as seen in the sentence "Her book report is due Thursday, but she hasn't read the book yet."

Hope this helps!