The right answer is the complementary DNA sequence of the radioactive DNA molecule (the radioactive molecule is called probe).
Here is the probe: 5' - gctagctaagccca - 3'
The sequence is 3' - cgatcgattcgggt - 5'
DNA probes are segments of DNA that are labeled (radioactive label in this case) to indicate the detection of homologous segments of DNA in tissue or culture samples.