Final answer:
Increased duration of acceleration and an upright sitting position can increase the severity of effects on the human body due to acceleration. Protective gear reduces injury, while younger age isn't a direct modifier. Falls can be safer on the Moon due to lower gravity.
Step-by-step explanation:
The factors that increase the severity of effects on the human body when experiencing acceleration are: a) Increased duration of acceleration, as it amplifies the impulse delivered to the body, and d) Sitting in an upright position, which can affect how forces are distributed across the body. Wearing protective gear typically decreases the severity of injuries, and younger age is not a direct modifier of the effects of acceleration.
On the Moon, a safe fall could be six times higher because the gravitational acceleration is about 1/6th that of Earth's. A softer surface reduces injury by increasing the duration of the impact, which spreads the force over a longer time, thus reducing its severity, as per answer a) from the referenced material.