Which aspect is critical when clearing a firearm?

When clearing a firearm, visual inspection of both the magazine and the chamber is indeed the critical aspect. This step ensures that the weapon is completely free of any ammunition, which is essential for safety. By visually inspecting the magazine, you confirm that it is empty, and by checking the chamber, you ensure that there is no round present that could accidentally discharge. This thorough verification process is necessary to prevent accidents and allows the individual to handle the firearm safely in a controlled environment.

The other options do not prioritize safety or proper procedure. Loading ammunition while attempting to clear a firearm contradicts the fundamental goal of this activity, which is to ensure it is safe. Aiming the weapon at a target during this process is also inappropriate, as this practice does not align with safe firearm handling and can increase the risk of accidents. Lastly, merely inspecting external parts fails to address the internal mechanisms where remaining rounds might be present, thus compromising safety further.