What shows that a vector is not empty?

Correct answer: Presence of desired insert DNA

The governing concept is the distinction between an empty vector and a recombinant vector. An empty vector contains its backbone and selectable elements but lacks the DNA fragment intended for cloning. Detecting the desired insert, for example by colony PCR, restriction digestion followed by gel electrophoresis, hybridization, or sequencing, demonstrates that the vector is not empty. Therefore option A is correct: the presence of the desired insert is the direct molecular evidence of a recombinant construct. A firm culture plate, clear water, or the smell of a host provides no information about vector structure or insert identity and cannot distinguish an empty plasmid from a recombinant one.