Which of the following would give the first positive indication that a changeover from one fuel tank to another is needed?

The fuel pressure warning is the first positive indication that a changeover from one fuel tank to another is needed due to its critical role in ensuring engine performance and safety. A drop in fuel pressure can signal that the tank currently in use may be running low or has become blocked, potentially leading to engine fuel starvation. This warning prompts immediate attention from the pilot or crew, as maintaining proper fuel pressure is essential for engine operation.

In contrast, the other options provide valuable information but may not serve as immediate indicators for a necessary fuel tank change. The fuel quantity indicator shows the amount of fuel remaining but may not reflect the situation until the tank is nearly empty. The fuel pressure gauge indicates real-time pressure but does not necessarily warn about an impending change unless a warning flag or threshold is reached. The fuel flow meter measures how much fuel is being consumed but doesn't directly indicate when a changeover is required, as flow can remain normal until the tank runs critically low. Thus, the fuel pressure warning is the most immediate and actionable indication for tank changeover.