Which component's placement must be checked to ensure alarm activation according to a plan?

The placement of the float is critical for alarm activation in a septic system because floats are typically used in combination with a control panel that monitors the water levels within a tank. The float switch senses the rising or falling water level and can activate an alarm if it indicates that the water is too high, indicating a potential issue such as a pump failure or blockage. Proper placement is key to ensuring that the float can accurately monitor the tank's level and signal the alarm system at the appropriate time.

In contrast, the timer, pump, and drainfield do serve essential functions in the septic system, but they do not primarily govern the alarm function in the same manner as the float. The timer controls cycles of operation, the pump is responsible for transporting effluent, and the drainfield provides a means for effluent to be treated and absorbed into the soil. Each of these components is important in the overall functioning of a septic system, yet they do not inherently activate alarms based on water levels the way a float switch does. Therefore, checking the placement of the float is crucial for ensuring proper alarm activation, making it the correct answer in this context.