For IFR day flight, which of the following is included in the additional required equipment?

In the context of IFR (Instrument Flight Rules) day flights, the additional required equipment includes a generator or alternator. This piece of equipment is critical because it ensures that there is a reliable source of electrical power for the aircraft's essential systems, including navigation and communication instruments, which are vital for safe instrument flying. Without a functioning generator or alternator, these instruments may fail, potentially compromising the safety of the flight.

Other equipment, while important for various reasons, does not fall under the essential requirements for IFR day operations. For instance, while a wind direction indicator or vertical speed indicator can provide useful information to the pilot, they are not specifically mandated for IFR operations. Similarly, a sounding device, although it might be necessary for certain types of flights or specific situations, is not categorized as essential equipment for IFR flying. Therefore, the generator or alternator stands out as the necessary equipment to ensure that all crucial systems remain functional throughout the flight.