Concept:Gamma radiation is a highly penetrating electromagnetic wave that occurs naturally as background radiation.
Explanation:Gamma rays cannot be seen by the naked eye, so Option A is incorrect.
Gamma rays have very high penetrating power and can pass through aluminium foil, so Option B is incorrect.
Gamma radiation itself is not used as a tracer; instead, radioactive isotopes that emit gamma rays are used, so Option D is incorrect.
Natural sources such as rocks, soil, and cosmic rays emit gamma radiation, which is always present in our surroundings.
Therefore, the correct statement is Option C.
Answer:Gamma radiation can be present naturally in our surroundings.