DNA probe is used in 

A probe is a radioactively labelled  DNA variable length (usually 100-1000 bases long) that can then be used in DNA samples to detect the presence of nucleotide sequences (the DNA target) that are complementary to the sequence in the probe. The probes are used in DNA finger printing for identification of sample DNA fragments sorted by gel electrophoresis and southern blotting. The radioactive probes are washed over the nylon surface to allow their joining to any DNA fragments of same composition thereby matching different DNA samples. Disease specific probes are used in antenatal diagnosis of presence of even a single gene defect in foetus. Probes specific to pathogenic bacteria are used for detection of presence of these pathogens in individuals. Correct answer is D.