Under what condition will HYD pump 1B not turn on?

The correct condition for why HYD pump 1B will not turn on is related to the status of the aircraft's electrical power systems. Specifically, when all generators are off, there is no electrical power available to operate the hydraulic pump. The hydraulic pumps require electricity to function, and without any operational generators to supply that power, the pump cannot activate.

In a scenario where hydraulic fluid is low, the system typically has safeguards that will either alert the crew or cause some degree of functionality to be retained, but this does not prevent the pump from attempting to engage. Similarly, whether the flaps are retracted or not tends to have no direct impact on the ability to activate the hydraulic pump; rather, flaps and hydraulic systems are often separate considerations within operational parameters. Lastly, when the aircraft is on the ground, there are often systems in place that allow hydraulic pumps to operate, especially for pre-flight checks or maintenance procedures, thus not inherently disabling HYD pump 1B.