What instruments can be used to determine that the alternator has failed?

The correct answer identifies the instruments associated with monitoring the electrical system in an aircraft. An ammeter provides a direct reading of the electrical current being produced by the alternator. If the alternator fails, the ammeter will show a discharge rather than a charge, indicating a problem with the alternator's ability to generate power.

Additionally, the low-voltage warning light serves as a crucial visual alert. When the alternator fails, and the battery begins to supply power without being recharged, the voltage can drop below a safe threshold, illuminating this warning light to indicate an electrical issue requiring immediate attention.

The other choices do not provide relevant information about the alternator's status. For instance, a speedometer and compass, an altimeter and vertical speed indicator, or a horizon indicator and attitude indicator are primarily concerned with flight performance and navigation rather than electrical system assessments. Thus, monitoring the ammeter and low-voltage warning light is essential for pilots to detect alterations in the electrical system’s performance effectively.