What phase of an explosion is most likely to produce injuries to the gas-containing organs...

The correct answer is A because primary blast injuries are caused by the blast wave itself , and that pressure wave most strongly affects gas-containing organs such as the lungs, ears, and bowel . Authoritative blast-injury references specifically state that primary blast injury damages gas-filled organs, while the other blast phases involve fragments, body displacement, burns, or other associated effects. ( NCBI )