Option (A) Probe is correct. According to the textbook context, during Southern hybridisation, VNTR Probes are used, which are defined as 'the labeled stretches of single-stranded DNA used to detect the presence of complementary target sequences.' This directly matches the question's description of a short stretch of DNA used to identify complementary sequences. VNTR (Option C) refers to 'variable number tandem repeat' - repetitive non-coding short DNA sequences. Minisatellite (Option D) is a type of VNTR with repeated sequences of 10-100 bp. Marker (Option B) is not defined as a detection tool for complementary sequences in the given context.