Final answer:
The M9 pistol does not have a safety selector for different firing modes; instead, it has a safety/decocking lever. The M16A2, M249, and M240G all have safety selectors to choose between different firing modes.
Step-by-step explanation:
The safety selector is a component on many firearms that allows the user to set the weapon to safe, semi-automatic, or fully automatic modes of fire. When examining the weapons listed in the question, the M9 pistol is the one without a safety selector that allows for selecting firing modes such as semi-automatic or automatic fire. The M9 does have a safety mechanism, but it functions only as a safety/decocking lever, which is different from a selector that chooses firing modes. In contrast, the M16A2, M249, and the M240G all have selectors that allow users to choose between different firing modes.