Flap position 1 will extend only the slats when the:

Flap position 1 is designed to extend only the leading-edge slats when the flap lever is moved from the zero position to position 1, regardless of whether the aircraft is on the ground or in flight. This configuration is particularly beneficial as it enhances the wing's lift characteristics at lower speeds, making it suitable for takeoff and initial climb phases.

In the context of the options provided, selecting this condition reflects the specific operational function of the flap system on the A320. The leading-edge slats are primarily designed to improve aerodynamic performance, and they deploy at this particular position to optimize the lift-to-drag ratio while maintaining stall margins during critical phases of flight.

The other options, while related to general aircraft operation, do not specifically describe the activation of slats in relation to flap settings. For example, specifying that flaps are set to position 1 for takeoff does not necessarily mean that only slats extend at that moment, as other conditions could apply. Similarly, discussing very slow airspeeds might allude to other flap settings or configurations, and mentioning the extension of speed brakes generally refers to a different operational function that does not impact the flap mechanism in this scenario.