For a projectile launched horizontally, the only acceleration acting on it (assuming we are neglecting air resistance) is due to gravity. This acceleration is directed downward and has a constant value of approximately \(9.81 , \text{m/s}^2\).
However, from the options provided, \(0 , \text{m/s}^2\) indicates no acceleration, which is incorrect for a projectile in free fall. The other values (5 m/s², 100 m/s², and 18 m/s²) are not relevant to the vertical motion due to gravity, which is what we care about in this context.
Since there is no option for \(9.81 , \text{m/s}^2\), the most appropriate answer would be:
0 m/s², but this doesn't accurately represent the actual vertical acceleration due to gravity. Thus, it may be beneficial to clarify that while everything else remains constant, the vertical acceleration is actually \(9.81 , \text{m/s}^2\) downward.