In blue-white screening, what do blue colonies represent?
cells that have not taken up the plasmid vector
cells with recombinant plasmids containing a new insert
cells containing empty plasmid vectors
cells with a non-functional lacz gene

The correct option is : cells containing empty plasmid vectors

Explanation:

The blue-white screen is a technique which involves the rapid identification of the recombinant bacteria in a vector-based molecular cloning experiment. In this method, a DNA ligated vector is inserted in a host cell which is viable for transformation and grown in presence of X-gal.    

The cells that are transformed with the vectors having the recombinant DNA produce the white colonies. Whereas, the cells transformed with empty vector i.e. the non-recombinant plasmids, produce the blue colonies.

atp is used for energy.