What must be done to make an airship descend?

To make an airship descend, it is essential to understand the principles of buoyancy and how an airship is controlled. The correct approach is to pump air into the ballonets. When air is pumped into the ballonets, it increases the pressure within the envelopes and causes the volume of lifting gas (usually helium or hydrogen) to be reduced. This reduction in lifting gas density decreases the overall buoyancy of the airship, leading to a descent.

By adding air to the ballonets, you effectively make the airship heavier relative to the surrounding atmosphere, causing it to sink. This process is part of the airship's method to control its altitude and maneuverability.

On the other hand, the other options would not achieve the same effect. Releasing air from the ballonets would typically allow more lifting gas to expand, increasing buoyancy and causing an ascent. Decreasing the weight of the airship may also lead to rising as it would become lighter than the surrounding air. Opening the gas valves would release the lifting gas, similarly making the airship less buoyant, but this is not a controlled method for descent compared to adjusting the ballast through the ballonets.