Nowadays it is possible to detect the mutated gene causing cancer by allowing a radioactive probe to hybridize its complementary DNA in a clone of cells, followed by its detection using autoradiography because :

Concept: 

• Autoradiography is a technique that makes use of an X-ray film to visualize molecules that have been radioactively labeled.
• A probe is a single-stranded DNA or RNA, tagged with a radioactive molecule.

​​Explanation:

• To identify the mutated gene probe is allowed to hybridize to its complementary DNA in a clone of cells followed by detection using autoradiography.
• The mutated gene will not appear on the photographic film, because the probe does not have complementarity with the mutated gene
• The probe will fail to bind to the mutated gene as it will not have complementary and hence does not appear on the photographic film.
• Therefore, the correct answer is option 4.