Step 1: Check option (A), indirect steam heating.
Retort sterilization of canned foods works by exposing sealed containers to saturated steam under pressure inside an autoclave. The steam never touches the food directly, it condenses on the outside of the can and gives up its latent heat through the can wall, which is why it is called indirect heating. This is the industry-standard method, so (A) is valid.
Step 2: Check option (B), forced convection hot air.
Air has a very poor heat transfer coefficient compared with condensing steam, so heating sealed cans with hot air alone would need impractically long process times to reach a safe centre temperature and is not used for commercial can sterilization. (B) is therefore rejected.
Step 3: Check option (C), blue light.
Visible blue light carries far too little energy to destroy bacterial spores inside a sealed, opaque metal or glass container, and it cannot even penetrate the container wall. Ultraviolet, not blue light, is used for surface disinfection, so (C) is rejected.
Step 4: Check option (D), direct flame contact.
Flame sterilizers exist for small, robust cans, where the sealed container is rotated through an open gas flame so the metal wall itself conducts heat rapidly to the contents, giving very short process times. This is a recognised, if specialised, industrial technique, so (D) is valid, giving the combination (A) and (D) only.
\[ \boxed{(A) and (D) only.} \]