Final answer:
The range of a projectile is zero when it is fired at a 90° angle or 0° angle.
Step-by-step explanation:
The range of a projectile is the horizontal distance it travels before hitting the ground. When the range is equal to zero, it means that the projectile lands at the same point from which it was launched, forming a straight vertical path. This occurs when the projectile is fired at a 90° angle (straight up or straight down) because it experiences no horizontal displacement.
Therefore, the correct answer is c. 90° or 0°.