Option 1: Both are inorganic compounds. This statement is incorrect. Bleaching powder, also known as calcium hypochlorite, is an inorganic compound that contains calcium, while DDT (Dichloro Diphenyl Trichloroethane) is an organic compound that does not contain calcium.

Option 2: Both are organic compounds. This statement is incorrect. Bleaching powder is an inorganic compound as mentioned above, while DDT is an organic compound.

Option 3: Both contain chlorine. This statement is correct. Both bleaching powder and DDT contain chlorine. Bleaching powder contains chlorine in the form of calcium hypochlorite, which is an effective bleaching agent and disinfectant. DDT contains chlorine atoms bonded to carbon atoms, which contribute to its insecticidal properties.

Option 4: Both contain calcium. This statement is incorrect. Bleaching powder contains calcium, as it is a compound composed of calcium and chlorine. However, DDT does not contain calcium.

In conclusion, the correct statement is option 3, which states that both bleaching powder and DDT contain chlorine.