Tryptophan binding to the repressor protein makes the proteins able to bind to the trp operon and block transcription. The lac operon is the opposite--allolactose prevents the repressor from binding to the operon and turns transcription on.
Work Step by Step
See pp. 362-363 for diagrams which show these facts. This question is trying to get you to see that the repressor system can be used to turn operons on or off depending on the nature of a particular operon.