Final answer:
The size of the helium balloon needed to lift a 75kg physics teacher increases with altitude. This is due to the decreased atmospheric pressure at higher altitudes causing the balloon to expand in order to maintain lift.
Step-by-step explanation:
The size of the helium balloon needed to lift a 75kg physics teacher varies with altitude as a result of changes in atmospheric pressure and volume. As the balloon gains altitude, the atmospheric pressure becomes less and the balloon expands according to Boyle's Law. To maintain enough lift for the constant weight of the physics teacher as the altitude changes, the balloon must have a greater volume at high altitudes compared to its volume at sea level.
Therefore, to answer the question, the size of the helium balloon needed to lift a 75kg physics teacher increases with altitude (Option A). This is because the atmospheric pressure decreases with altitude, causing the helium balloon to expand to maintain the same lifting force.